Referring to canned food, what does a bulging can mean?

I agree with all the above answers, NEVER, NEVER, eat food from a swollen can. But I would take it back to the store to exchange for another can of product. Most stores will gladly give you a replacement for damaged goods.

Some bacteria can survive the canning process, and can release gases, which build up and make the can bulge. Clostridium Boltulinum is the main culprit for this issue, and is very poisonous and releases toxins as well.
Never eat food from a can that has bulged out.

It means no one in my house is eating it and it's going in the trash.

It means the sides are protuding from the horrible poisonous gases inside. It's time to get rid of it with plastic gloves and safety glasses.

Pick it up, put it in a plastic bag and throw it out. The food inside is bad and it's going to blow sooner or later. Get rid of it.

This is a sign of a spoiled canned food. Foods from such cans are not safe to eat, even after cooking.

source: http://msucares.com/health/food_safety/foodfaq39.html

It means, THROW THE FOOD AWAY! And if you re-use the jar, make sure you sterilize it well. Throw away the lid, though, of course.

Fermentation

+5

Bulging of canned foods occurs due to the action of various microorganisms. Spore forming bacteria, e.g., Clostridium, Bacillus represent the most important group of canned food spoiling microorganisms because of their heat resistant nature (thermophilic nature). In addition, there are other microorganisms which are not heat resistant (mesophilic) but enter through the leakage of the container during cooling and spoil the food.
But bulging is mainly due to the Clostridium spp & Bocillus spp.