On a checkerboard, black squares alternate with white ones. As a verb, alternate means to do something in turns. As a noun, an alternate is a replacement.
While the noun and verb seem not to mean the same thing, think of it this way: If you are brought on in a game to be someone's alternate, you are there to take their turn. Alternate can also be used as an adjective, meaning different or other. An alternate view of history is one that looks at the past from an uncommon perspective. Pay attention to the pronunciation of alternate: as a verb the end is pronounced "-nāt" and as a noun or adjective "-nət."