Autocomplete feature helps users by automatically filling the content of cell when users starting entering the first couple of letters in the Cell. The automatic entry is picked from the previously entered values for the same column.
Let us say you have got the following list of entries in one of the column
Arizona
Alabama
California
Florida
Indiana
Now when you start typing Ar in the cell below Indiana, Spreadsheet will automatically display Arizona based on the entries done earlier for that column. And when you press enter this value will be automatically filled for that cell.
You can enable Autocomplete feature in Google Spreadsheet by clicking the Tools menu and selecting Enable AutoComplete from list of available menus.
Remember that if there is a empty cell before the active cell then Autocomplete feature will try to pick from the below column values.