It’s not… Not anymore. Though the at symbol (“@”) was first used to mean “at the rate of” —to have an easy way such that e.g. “4 bottles @ $1” meant “$1 each”, not $1 for the whole— the symbol’s meaning has changed thanks to one man: Raymond Samuel Tomlinson.
Tomlinson, then working at BBN technologies, was hired by the U.S. government in 1971 to develop ARPANET, the rudimentary version of the internet. One of Tomlinson’s big challenges was to uniquely identify each individual working on this project; but many users logged in to the same computer, and it all connected to the main system. This meant that Tomlinson could identify computers, but not individuals.
Tomlinson knew that the unique ID must include the sender’s name and the name of the computer from which it was sent. As almost all other symbols had some meaning in programming, Tomlinson set his focus on the at symbol.