A cross slide vise is a vise with a three lead screws. One lead screw is used to open and close the vise jaws. The second lead screw moves both vise jaws along the x axis (left to right). Third lead screw moves both vise jaws along the y axis (up and down).
A cross slide vise works very similarly to a milling table. But it has some advantages and disadvantages compared to a milling table.
Most milling machines will come with their own sliding table, but these tables are not meant to be removed from the machine. These tables tend to be much more precise than cross slide vises. The difference is especially noticeable when comparing milling machine tables with cheap cross slide vises.
A cross slide vise is useful in situations where you need the cross sliding functionality of a mill table, somewhere other than your mill.
Side note: I see a lot of people trying to make a poor mans mill on youtube by using a cross slide vise with a drill press that has a milling bit attached to it. This is dangerous. A drill press is not made to withstand lateral forces like you get when you mill a piece of stock.
Trying to use your drill press as a mill with the help of a cross slide vise will end up with either you or your equipment being damaged.
A cross slide vise IS helpful when you need to drill holes more precisely, The ability to move your stock along the xy plane while working allows you to drill your hole exactly where you need to. You will not get the precision that you would get with a mill table, but for most projects you don’t need that level of precision.
A cross slide vise is useful tool that has a place in every workshop.