Because the muscles in our throat and mouth are under a lot of stress, it is important to make sure they don't tire out. Just like bodybuilders and sportspeople warm their muscles up before a workout, singers need to make sure they warm up their voices before attempting strenuous notes or before a long session of vocal exercises.
Different singers have different ways of warming up their voices, and depending your own voice, you may be given some personalized voice exercises to practice. But for most, a general set of warm up exercises will work just fine.
The seven warm up exercises in this infographic are a good start, and cover all the basic muscles that a singer needs to stretch every before a long day of singing, or just to keep in practice.
Infographic by: www.musicademy.com