Going to point out a piece that seems to get overlooked: if there's a concept/idea you're not familiar with and it's a relatively new concept not found in textbooks, it's probably introduced and explored in one of the citations. Not all the citations will be useful but they are a basic dependency map of what you need to know to follow a field.

Secondly, depending on the age of the citation it is highly likely you can find the paper (or a draft version at least) for free on the author's website or they author will email it to you if you ask kindly.