Until surprisingly recently, there was no built-in way to make a rectangular element on a webpage have rounded corners. People had to use all sorts of dirty hacks to make a final product that "looked like" it was a native, built-in feature. Usually in the old days it basically amounted to various forms of "putting a picture of a rounded corner" (often a gif) in each of the corners of the element you were putting on the page.

There were a lot of different ways to do that, but one of them was to use "tables" - tables these days are usually only used for what their genuine, semantic intent is: for drawing a literally spreadsheet-like table of data. But back in the earliest days of the web, they were the only controllable way to visually lay things out on any kind of grid, so despite the fact that they were "supposed" to have nothing to do with visual layout, they'd get used all the time for that - often getting used to do visual borders and stuff.

So to do rounded corners, you'd basically make a table that was a 3x3 grid. In the corner elements of the grid, you'd have tiny pictures of rounded corners; in the side elements of the grid you'd basically have nothing (they'd be really skinny elements, either very wide or tall). Then the middle element in the grid would be gigantic, and would hold your actual content.