Chris Messina
The slash ("/" aka "the slasher") is used to set off metadata from the rest of a tweet.
Metadata to a tweet has often been surrounded by parentheses to set it off from the main body of the tweet. While this works, it uses one character too many. The slasher is a convenient and compact way to indicate the start of metadata.
To use the slasher, just prefix your tweet's metadata with the slash!