When you select text with your mouse the background changes color: with my preferences it becomes dark yellow. This background color changes back to white when you unselect the text by clicking with your mouse again.

When you press the Alt (Meta) key while selecting text the background of the selected text changes to light yellow. This color is permanent, it stays light yellow even when you click somewhere else with your mouse. The background only changes to white again if you click on the light yellow text while simultaneously pressing the Alt (Meta) key.