What is the Novikov Self-Consistency Principle?
Also known as the "no-stupidity" principle, this is a rule of time travel that states any events that occur through time travel must not create paradoxes or logical inconsistencies. In other words, if you try to kill your own grandfather, it's not going to work. He'll just be all, "Uh, no, I'm your grandpa, you're my grandchild, and we're both still going to have a nice cup of tea."
What is the Bootstrap Problem?
A classic problem in time travel, the Bootstrap Problem asks: if a time traveler creates a new timeline through their actions, can they ever return to their original timeline? If so, how do they get back? And if not, how do they explain the whole "I've been to the past, but I'm stuck in the future" thing to their therapist?