See https://en.wikipedia.org/wiki/Topological_order.
And, as always(?), 3Blue1Brown also contributes https://www.youtube.com/watch?v=IQqtsm-bBRU "This open problem taught me what topology is" I'll warn that while it is illustrative, and only 28 minutes, it's some heady stuff
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And, as always(?), 3Blue1Brown also contributes https://www.youtube.com/watch?v=IQqtsm-bBRU "This open problem taught me what topology is" I'll warn that while it is illustrative, and only 28 minutes, it's some heady stuff