The anthropic principle is the idea that any condition in the universe which must be true in order for us to exist is unremarkable; the fact that we're observing the universe means that any observations which are necessary for our existence can't be used to make definitive claims about existence.
So, if it were true that life on this planet would be impossible if the earth were thirty meters farther from the sun (it's not), the position of the earth in relation to the sun is completely unremarkable, and cannot be used to extrapolate on the positions of other planets in relation to the stars they orbit.
(If these factors weren't in place, we wouldn't be here observing them; we are observing them, therefore these factors aren't predictive in describing the universe.)
This is a very subtle argument, and has a lot of implications.
If an asteroid hitting the planet were necessary to wipe out the dinosaurs, and the dinosaurs must have been wiped out in order for us to exist, an asteroid hitting the planet is entirely unremarkable, and cannot be used to predict the likelihood of asteroids hitting the planet in the future. Likewise, if a particular -infrequency- of asteroids hitting our planet is necessary for life, we cannot predict with any likelihood that this infrequency will continue to be the case.
It has weirder implications. If climate changes in the past were necessary for our existence, we cannot say with any certainty climate changes will ever happen again. If evolution were necessary for our existence, we cannot say evolution happens anywhere else, including if life occurs on other planets. If a stable climate were necessary for our existence, we cannot say the climate will remain stable in the future.
The anthropic principle means that anything that must be true for our existence -may not continue to be true- in the future, -no matter how reliable- they've been until now.
The universe isn't nearly as friendly as it looks. Considering how inhospitable it looks, that's saying something.