The quad muscle is responsible for extending your knee and flexing your hip. The rectus femoris muscle is a 2 joint muscle and should therefore be strengthened when addressing hip, knee, or low back issues.
The exercise pictured above from HEP2GO is the Straight leg raise in supine, also referred to as SLR, and should be performed so your muscle becomes tired within 15 repetitions. Rest 30 seconds and repeat for a total of 3 times. Perform daily. Stop if you have pain, cannot perform the exercise the same way, or achieve fatigue in the muscle.
If you can perform this exercise without achieving fatigue, speak to your therapist about how to progress this safely and efficiently to achieve the best outcomes.