tire problems

I have been getting a tire defect warning on my 2001 740il. The tires look ok. what does this warning usually mean? I thought it might be some type of alignment issue.

Your tire pressure is low. Take it to a gas station to get them filled up. I forget if you have to reset the monitior or not. Also the tire that has the lowest PSI you might wana check for a nail or something that could cause a leak.