The built-in command bg sends a suspended job to run in the background. With no arguments, bg operates on the most recently suspended job. To specify a particular job (shown by the jobs command), supply the job number preceded by a percent sign:

$ bg %2

Some types of interactive jobs cannot remain in the background—for instance, if they are waiting for input. If you try, the shell will suspend the job and display:

[2]+  Stopped            command line here

You can now resume the job (with fg) and continue.