Complete Selection reads your selection, which should be a single word or an insertion point following a word, and looks for possible completions in the list of all currently defined functions and variables.
Complete Selection also provides completions for keyboard aliases and full names of special characters, such as \[Alpha] and a.
When there is only one possible completion, Mathematica automatically inserts the additional characters to finish what you have typed. Otherwise, a popup menu lists the various possible completions. To insert one of them, click the completion you want or type another letter to reduce the list of possible completions.
Press Backspace to cancel the popup menu.
If the list is long, you can scroll through the menu by using the mouse or the arrow keys.