They are tiny little muscles that twitching. Normally, they only move in unison with other muscles so you never notice them. The twitching won't hurt your blood vessels. It's usually an indication of an imbalance of something in you body.
The twitching can indicate a lack potasium in your body. Potasium helps insulate your nerves to keep the signals from going to the wrong places. It also help to keep neurons from misfiring. When that happens, a small muscle seemd to pulse by it's self.
Try eating a banana everyday to get potasium.
Stress also causes misfiring of nerves in your brain which can cause a muscle or group of muscles to twitch. If you have been under a lot of stress, it's best to discuss it with your family doctor. Too much stress is a bad thing.