No this is not texting shorthand for accessing your inner child or a comment on the dismal state of last night's date. B12 is a vitamin we need to make red blood cells. Without enough of it, you can get anemia. Since we really only get it naturally by eating animal foods, when someone is avoiding those foods, they put themselves at risk and might start feeling tired or looking pale and then be prodded to eat meat. If you'd rather not then just get a supplement.
One of the biggest reasons for B12 deficiency that can lead to all sorts of problems beyond lack of energy, (Asthma, depression, MS, just to name a few.) is that it can be challenging to get it through digestion and many people have compromised digestion.
Digestion is such a gigantic key to moods and well being, it's a good idea to take care of it and find out what makes your 30 feet from mouth to rear, the healthiest it can be. A B12 supplement might be a good start.