What to check before you choose
Two cautions are worth more than any tool recommendation. First, AI summaries and explanations are confidently wrong often enough that you should never let one be your only source for something you will be examined on — check it against your actual course material. Second, most institutions now have explicit policies on AI use in assessed work, and those policies vary between departments; using a tool that is fine for revision but not for submission is an easy mistake to make. Used for practice and recall rather than for producing work you hand in, these tools are genuinely useful.