I am really struggling in understanding this concept (mainly because I'm doing my math course online, and it's pre-calculus and not calculus), and I couldn't find any thing on this concept at all online after multiple tries on different search engines. I know it has something to do with instantaneous velocity (instantaneous rate of change), but my book doesn't explain any further than that.

If anyone can point me in the direction(helpful links) or explain the concepts to me, I'd greatly appreciate it. Thank you in advance.