When a command is running in the background, because you started it with & at the end (example: top & or because you put it in the background with the bg command, you can put it to the foreground using fg.

Running

fg

will resume to the foreground the last job that was suspended.

You can also specify which job you want to resume to the foreground passing the job number, which you can get using the jobs command.

Running fg 2 will resume job #2:

The fg command works on Linux, macOS, WSL, and anywhere you have a UNIX environment