If it falls to 9.5 or less, the battery is on its way out and will need replacement. If it remains above 10 volts, it’s good. Once the bike starts, watch the reading at a fast idle: the voltage should read close to 14 to 14.5 volts. If the reading stays at the no-load voltage or below, the charging system is on the fritz. If everything looks good on the meter, you need to ride more often, or leave the bike on a charger between rides. By the same token, if this situation occurs on a bike that’s ridden often, you’ll have to delve a little deeper to find out what’s killing the battery when the bike is parked.
Granted, the above examples are relatively simple problems, intended primarily to illustrate how DVOMs and test lights, along with the shop manual and a little logic, can track down an electrical fault. More involved problems, for example, those dealing with voltage drops, high circuit resistances, or parasitic draws, will require commensurate effort to solve. My suggestion, should you have any real interest in that direction, would be to read through the appropriate manuals on the subject. I’ve listed some of my favorites in the sources section.