What does major tenets mean?

Answer 1

It means the most important beliefs of, in this case, Buddhism. Buddhism isn't really about "beliefs" it's about insights into the way the world works that you can see for yourself, but you do have to take a little step on "faith" to test its tenets and try them for yourself. The Four Noble Truths are probably the most basic tenets: That every being that can suffer does suffer at some point and in some ways, that we humans cause our own suffering, that the cause of our suffering is clinging (to a wide variety of things, most notably mistaken beliefs), and that the practice described by the Buddha (known as the 8-fold path) is the way to end the clinging and therefore the suffering.

Other major tenets of Buddhism is that all things are interrelated = nothing stands alone; and that nothing lasts forever.