Jam has several distinct meanings:

1. A sweet, thick spread made by cooking fruit with sugar until it becomes soft and sticky. Strawberry jam on toast is a classic breakfast. Unlike jelly, which is smooth and clear, jam contains pieces of fruit. Making jam preserves fruit so it lasts through winter, a technique people have used for centuries. Your grandmother might spend a summer afternoon making raspberry jam from fresh berries, pouring the hot mixture into jars that seal with a satisfying pop.
2. To push or squeeze something forcefully into a tight space. You might jam too many books into your backpack, making the zipper hard to close. A printer jams when paper gets stuck inside. When a door jams, it won't open or close smoothly. This kind of jamming usually happens when you're in a hurry and things don't cooperate.
3. To play music together in an informal, spontaneous way. Musicians jam when they create music on the spot without written music or rehearsal. A jam session might happen when friends bring guitars to a campfire and start playing whatever comes to mind.
4. An unwanted interruption of signals or radio waves. During World War II, countries would jam enemy radio broadcasts so people couldn't hear them clearly.