You are about to start some exercise program for getting better abs, but it is still difficult for you to make a sign abdominal crunch. Then the plank is a simple way to increase your upper body muscle endurance which will be the base for some further more complex ab exercises.
If your upper body muscles are generally weak and you decide to start some aggressive ab muscles program without having the proper technique learned it is very likely that you will apply huge stress on your spine which may result with pain in the lumbar area.
The plank is one of the best exercises that strengthens the core, the deep upper body muscles. While performing this exercises instead of moving parts of your body you should retain the same body position for longer time, but there are also some variations of this exercise in which are included movements. Basically it is a static exercise that does not require any equipment and is very practical and efficient open for modifications and improvisation. The plank is more efficient than dynamic exercises that usually activate only the superficial muscles and their primary focus is on a smaller muscle group. Unlike these exercises the plank isometrically activates the muscles on the back, chest, arms and thighs which is the base for better durability and potentiation on whole upper body. Once you improve the upper body stabilization you will have better base for performing different types of exercises like push-ups, different variations of abs and back exercises.
There are various combinations of the plank exercise that target different muscle groups.
First, start with 20-60 sec per position, 3-6 sets and as you progress and adapt you can increase the number of sets or the time for each position.