Linux – What does the character , mean in bash

From help set:

  -T  If set, the DEBUG trap is inherited by shell functions.

So if you use trap to invoke a function on DEBUG (that is, almost before every command in a shell script) and then invoke another shell script, the trapping will occur in that script as well. Without this option, the trap will not exist in the subshell and the script invoked in it will run untrapped.

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.