Anything canned is stored in a jar or can. It’s usually food, but prerecorded music is also canned. And, if you get fired from a job, you’ve been canned. Don’t cry, though, it’s better than being stuck in a jar.
When someone talks about "canned goods," they mean all the food you can buy that comes in a can, like canned green beans and canned pineapple. Home-preserved food is also canned, even if it's in a glass jar. This adjective takes on a derogatory slant when it describes music that's recorded rather than played live, or an answer that sounds prepared rather than real.