Placenta previa is where the placenta covers part or all of the cervix. There are grades 1-4 depending on how much the cervix is covered. An ultrasound is used to asses this.
In early pg you can have a low.lying placenta. Which can 'move' which really means as your uterus grows the placenta gets further away from your cervix.
Main thing to watch for is bleeding, most commonly without pain. So any bleeding needs to get checked. If your cervix dilates at all your placenta can abrupt (come away from you uterus).