"I use hundreds, thousands of various skillful means, such as different interpretations, indications, explanations, illustrations. It is not by reasoning that the way is to be found: it is beyond the pale of reasoning..."
--Shakyamuni Buddha, The Lotus Sutra
The Buddha spoke to many people in his lifetime, and he tailored his message to be as helpful as possible to each audience. When he spoke to his most dedicated and understanding followers, he gave them the highest level of abstract explanation they could swallow. When he spoke to skeptics, he encouraged them to put his teachings to the test of their own experience. When he spoke to ordinary citizens, he packaged his message in terms of their existing beliefs. He used stories and metaphors when he felt those would be most helpful. In short, he was a tactful teacher. The Buddha's use of tact is what he called "skillful means."
When people dive into the study of Buddhism without understanding skillful means, without taking into account the context and intention of each teaching, the overall message is not clear. It's kind of like reading a science article on wikipedia: one section is written at a 5th grade level, the next is written for grad students in quantum mechanics, and the overall meaning is lost. When the thousands of teachings of Buddhism are approached in the same haphazard way, there are many apparent contradictions in the teachings. However, these contradictions are resolved when they are put into context.
Here's an example: In the time and place of Buddha's teachings, about 400 BC in India, the culture widely believed that each person had a soul that would undergo reincarnation, and good deeds in this life would lead to reincarnation in a better state of existence, while bad deeds in this life would lead to rebirth in a state of existence with more suffering. When speaking to the general public, the Buddha explained his teachings in terms of these ideas because it would lead ordinary, selfish people to discover the joy of altruism. Thus it's possible to find many sutras in which the Buddha says that rebirth is real. However, when the Buddha spoke to his earnest but idealistic disciples, he said that rebirth is not real. He warned them that they should not believe in rebirth because it would cause them to believe that they have real ego-souls, and believing in the reality of the ego-soul would lead them back to selfishness, attachment, and suffering.
When speaking to a crowd of his wisest disciples in the Lankavatara Sutra, the Buddha gave a third interpretation of rebirth. He said that rebirth--along with all other phenomena--is neither real nor not real. He told these disciples to not get hung up on his other teachings; he was just using skillful means. In the other situations, he explained, he taught people according to their ability to understand. He said that, although the truth is that there is neither rebirth nor not rebirth, some of his disciples get attached to the selfish concern of their reincarnation. For them, they are better off starting with the concept of no-ego and no-rebirth. For other people, who are selfish, foolish, or superstitious, or who interpret the concept of no-ego and no-rebirth nihilistically, the Buddha said it would be better for them to believe in the reality of ego, soul, and rebirth so that they do not wallow in despair and lose sight of the teaching's ultimate purpose.
So there are 3 levels of interpretation: reality, no-reality, and neither reality nor no-reality. It's like the different interpretations of physics in high school, college, and grad school. In high school, we're taught that electrons behave like particles. In college we learn that electrons behave like waves and particles. In grad school they teach that electrons behave so crazily that no one even dares to say what they are. As with the Buddha's teachings, increasing levels of abstraction build up to a more refined and precise understanding that eventually surpasses the capacity of language. But each level is useful and serves a purpose. If you're just getting comfortable with high school physics, don't worry about quantum electrodynamics. If you're taking quantum, don't get hung up when the book disagrees with your high school physics teacher. Same thing with the sutras.
In Buddhism, they don't label the sutras according to grade or suggested age level, but they always start off by describing the audience the sutra was addressed to. That can give you an idea of how much subtlety is in the message, and what sort of skillful means are being employed. You might find one level speaks to you more than another. Don't worry about whether you're reading the most advanced stuff--don't make it an ego thing. All the teachings are here for our benefit. They should simply be used in the way that is most useful.