So you have a program running, and you press CTRL+Z. This stops the job and gives you a prompt.
You then type bg and the job continues running in the background.
You go and do other things. Now you want to bring the job to the foreground.
You type jobs and see a list of the background jobs, in the format [Job#] PID.
So you type fg Job# and bring the process to the front.
If there's only one background job, fg by itself will bring it up (no need for a job number).