It tells the system to run the command in the background. From the bash man page on my system:
If a command is terminated by the control operator &, the shell executes the command in the background in a subshell. The shell does
not wait for the command to finish, and the return status is 0. Commands separated by a ; are executed sequentially; the shell waits
for each command to terminate in turn. The return status is the exit status of the last command executed.