JAM has 2 meanings
To pitch near a batter's hands
A baseball term that commonly refers to a pitcher who throws a pitch near the batter's hands, which limits how far the batter can hit the ball; can also be used to describe the trouble a pitcher can get into, such as letting batters get to second and third base with no outs, during an inning.
Lopez is able to jam batters with his pitch that runs up at the hitter.
If you need to pause an activity, such as a game or conversation, you can send "JAM" to the other gamers, partners, etc., to say "just a minute." It's similar to "hold on a second" (HOAS) and a little bit longer than the JAS acronym.
Most people use JAM when time is a factor when texting or messaging online in a real-time conversation or game. For example, if you are texting with your friend about school but your mom needs to talk to you briefly, you might text your friend, "JAM. Gotta talk to the mother." Or, if you are gaming and need to take a restroom break, you might message other gamers, "JAM, gotta go take a leak."