I love doing planks because they work your entire core--not just your external abdominal muscles but all of your abs and your back muscles as well. They're challenging, for sure; don't be discouraged if you need a break after a a few seconds or if you need to keep your knees on the ground. Any discomfort in the lower back is a red flag!
The more you do these, the longer you'll be able to hold them.
There are some common mistakes people make, like rounding the shoulders and upper back; avoid this by "setting" your shoulder blades; in other words, get your shoulders back and think about bringing the shoulder blades together.
Think about your body being as flat as a table top.
Check out this 1-minute video to learn more!