An AC is typically a smaller piece of glass which has one female glass joint, and one male glass joint. They are designed to pair with other glass such as bubblers or j-hook mouthpieces. Originally they were made to catch the ash from combustion smoking so the main bong water doesn't get dirty as fast. But people have discovered they also work great as mini portable bubblers
Here's an example of an Ash-Catcher connected to a larger waterpipe:
And here is an Ash-Catcher connected to a J-Hook to make a mini portable bubbler: