Hi, Ashley! Planks are a static (non-moving) position that works the entire core of your body (abdominals and erector spinae) and are an excellent complement to standard supine (face up) abdominal crunch work and weighted back work.
A standard prone (face-down) plank looks just like a push-up position with the arms straight, either off the toes or the knees (off the toes is more challenging), at the top of the push-up; a prone hover-plank has you on your elbows rather than on the hands with the arms straight. You are using your core muscles to keep your body perfectly flat (no hike or sag at the hip joint).
A standard supine (face up) plank has you, well, face-up, with your arms straight down and your heels on the floor; you are using the core muscles again to keep the body perfectly flat with no hike or sag to the hip joint.
Additional challenge is inserted by varying one leg position. There are also side planks to target the obliques more.
HTH -
A-Jock