This post is an explanation and notes of the anthropic principle to help me better wrap my head around the concept, which is a combination of partially regurgitated information from YT videos and Wikipedia, and admittedly I still don't fully understand all of this. Essentially, it states that the very act observation can only occur in a universe that's capable of developing intelligent life that can make those observations. This explains why the universe has the age and physical constants it does that are necessary for conscious life to emerge, since if either had been different humans would obviously not be around to make observations. This also serves as an explanation for why the universe appears to be finely tuned for the existence of life, often cited as an argument for intelligent design.
Nick Bostrom, who has contributed a lot to the concept, delves into this a great deal in his book Anthropic Bias, where he divides the anthropic principle into "weak" and "strong" forms.
According to Brandon Carter, the weak anthropic principle states that the universe's "fine tuning" is actually the result of survivorship bias, a form of selection bias of concentrating on people or things that made it past some selection process while overlooking those that did not due to their lack of visibility. We (the human observers) are necessarily at a privileged location within the universe in both space and time in that it is compatible with the existence of human observers.
John Barrow and Frank Tipler differ from Carter in their book The Anthropic Cosmological Principle, and basically restrict the principle to carbon based life as opposed to just observers, spacetime dimensions, and the cosmological constant.
The strong anthropic principle states that the universe is compelled to eventually have conscious life emerge within it. Here Carter states that the universe and the physical constants (ie gravity, speed of light, etc) of which it depends, must be such as to admit the creation of observes within it at some states.
Here, Barrow and Tipler state that "the universe must have those properties which allow life to develop within it at some stage in it's history" which is similar to Carter's view. Unlike Carter, however, "must" (or "required") is an imperative as outlined by Barrow and Tipler. They use language like "There exists one possible universe 'designed' with the goal of generating sustaining 'obververs'." This can really be reduced to an intelligent design argument. Barrow and Tipler also argue that observers are necessary to bring the universe into existence in order for quantum mechanics to make sense. This claim isn't testable as we aren't able to observe other universes for intelligent life other than our own, therefore unfalsifiable.
There are applications in physics for the anthropic principle, however for me, I really enjoy thinking about it as a thought experiment.
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