A "blind spot" is an area alongside your vehicle that you can't see when using any of your mirrors. It's important to check these blind spots to ensure your side gates are open, particularly if you are going to change lanes.
A quick glance over your shoulder, or "shoulder check," can allow you to see the entire side gate area of your vehicle, which could be a blind spot.
Shoulder check your blind spot for the gate you are driving into. For the left side, turn your head left until you are looking along your shoulder. You are checking to see if there is a vehicle closing your gate. If there is something there, do not try to move into that space. To check the right side, you need to turn your head to the right.
For your own safety and the safety of other motorists, it is very important that you don't drive in other drivers' blind spots. Move forward, or drop back, as soon as you notice this situation. The blind spot can be significantly reduced if your mirrors are properly adjusted.