Your quadriceps are a group of 4 muscles (rectus femoris, vastus medialis, vastus intermedius, and vastus lateralis) that together work to straighten the knee, when activated.
If you do not have full knee flexion (a minimum of 120, but depending on what you need to do, it could be up to 150), you can assess whether this is due to an issue within the joint itself or if it simply can be attributed to tight quads. This can be assessed by laying on your stomach and using a contract/relax strategy to see if you can gain further knee flexion (a tutorial on contract/relax stretching for the quads: https://www.youtube.com/watch?v=tO4PH3FsTK0)
If you determine that you have tight quads, or if you have some soreness from use of the quads, the standing quad stretch is a quick, easy way to stretch. You just need a doorway, table, or something to balance with. Bend the knee and grab the foot with the hand on the same side. Ensure that the thigh is directly below the hip, and your hips are not shifted from parallel. You can gently tuck your tailbone slightly for an increased stretch.
As with any muscle stretch, hold a minimum of 30 seconds.