Popularized by Mark Rippetoe, this "movement blocking" exercise works to improve your squat form. Just keep your toes right in front of PVC pipes, half foam rollers, or anything like that. Don't let your knees touch the object as you squat.
Note: This drill won't guarantee direct carryover to training settings involving moderate to high intensity workloads. Strength is specific to direction, load, exercise variation etc., so train as such.
Simply view this drill as a foundational movement technique to help make learning the squat easier with strictly internal resistance. Afterwards, add introductory squat exercises such as the front squat, low-load back squat, and goblet squat variations to add in external resistance appropriately.