It’s undercharged. There could be a number of reasons why this has happened, like accidentally leaving headlights on, doing too many short journeys, an electrical issue in your car that’s draining it, or it could just be an old battery that needs replacing. If it’s nearing the five year mark, look into getting a new one.
Disclaimer: Every car battery is different. Without actually examining the car, it can be hard to figure out exactly what’s actually going on with the battery. If you’re having trouble with yours, you’ll get the best advice from a professional. And always get it looked at as soon as possible, so that small problems don’t get the chance to grow into bigger ones. Never drive your car unless you’re certain it’s safe to do so.