Fixing a reference in Excel

In this video, you talk about fixing row references and in the earlier video, you talked about both row and column references used $ symbol.
I did not get what does it mean. And how an excel table can change its position.
And if it changes the position of how reference is able to trace it. https://learn.365datascience.com/courses/introduction-to-microsoft-excel/index-match-and-their-combination-the-perfect-substitute-for-vlookup