If your vertices turn yellow and red while selecting them then you have “soft select” turned on.
Sometimes you want more than just one vertex to move as you want to manipulate or “deform” an area.
You can select the B key on your keyboard to activate this feature. If you want to turn it off, just hit the B key again ( it is a shortcut key )
By holding down the B key while left clicking ( hold and drag ) you can increase or decrease the affected area.
Please check out my channel at https://www.youtube.com/user/MikeHermes65