It's hard to tell without seeing what else it's connected to. However if I had to guess, it looks like a portion of your subframe.
A subframe is a piece of metal that typically stretches from one side of your car to the other and is bolted to the actual frame of your car. Its task is to support the weight of your engine and transmission.
Although I can't see the crack (it might help to take a closer, more detailed photo of the crack itself) I would say that if it involves the subframe and the crack is substantial, yes, it is a bad thing. If, when driving, the subframe cracks in half, there's very little support for a ~300+lb smorgasbord of cast iron and steel parts.
With that said, don't be TOO worried. Bring it somewhere like a Monro or something and have them lift it up and get their opinion (don't necessary have them do the work, their labor rates are high and depending on the mechanic, may not be qualified). Bottom line, get it up on a lift, get someone's opinion and take many photos of it. There are a lot of forums about jeeps or cars in general who can give you better insight.