Some xkcd comics are classics - my gotos are the one about ISO dates (https://xkcd.com/1179/) and password strength (https://xkcd.com/936/). Both of these were found by search Google 'xkcd (dates|passwords)', and having them be the first result.
Explain XKCD is great if you know roughly what topic was covered by a comic, but can't quite find the URL. https://www.explainxkcd.com/