It's placing the baby on it's stomach ("tummy") so he can practice raising his neck to strengthen those muscles. It's the first step to being able to roll over and sit up.
The reason tummy time is recommended is because now it's encouraged to have babies sleep on their backs to avoid SIDS (sudden infant death syndrome). The down side is they don't get time on their stomachs to work those muscles, and so you have to put them on their stomachs (and supervise them).
Fun other fact: the vertebrate digestive system evolved to "hang downward"—i.e. for gravity to aid digestion by pulling matter toward your anterior.
If you ever feel gassy, one of the best solutions is to lay on your stomach and then slightly elevate your butt until you've created a pathway where the gas can exit by rising.
Similarly, if you ever feel constipated or have acid reflux, try just getting down on your elbows and knees (on your bed or another soft surface) and then relaxing your core to let your stomach just hang down. Picture imitating the body position of a cow. Everything should run in the right direction again after 10 or 20 minutes of this (bring a book.)
I bring this up because, of course, babies shouldn't be any different. If a baby—especially after weening—is colicky and looks to be in pain, it might be gassy bloating that it has no idea how to resolve. Flip them over.
