Your bird uses many ways to communicate with you. This may be through singing and squawking, avoiding eye contact or how it holds its wings. In general, if your bird's wings are drooping it is trying to tell you it's not feeling well. Be especially concerned if your bird won't move from the cage floor. You should get your bird to the vet as soon as possible.
Your bird may also let its wings droop if it is too hot. Most birds tolerate heat better than cold, but they should never be kept in the path of a heat duct or in direct sun. By drooping its wings your bird is trying to dissipate heat by getting air to flow under its wings. Move your bird to a cooler location to see if its wings perk up.
If your bird is drooping just one wing this is a strong indication that the drooped wing has been injured. It may have flown into something or done damage to itself in its cage. Regardless of how it happened, you should assume the wing is broken and take your bird to the vet immediately.
If your bird is young and is just learning how to adjust its wings it may let them droop from time to time because it hasn't quite figured out how to hold the wings tight to its body. If your bird gets wet it may droop its wings for a time to let them dry. Also, a bird that's been playing hard may droop its wings just to rest. If your bird doesn't stop drooping its wings once it dries or rests for a while, you should consider taking it to your vet.