How to keep a table header row stable while the user scrolls using CSS

I have a php page that pulls a query and displays the results in a table.

I want to have it so that the table's header row, containing the column names stays static while the user scrolls vertically but moves with the table when the user scrolls horizontally.

I know it's possible becaues I've seen websites that do this, just can't figure out how to do it myself.  I assume it's doen with the CSS Position attribute somehow.