The embouchure is the seal that your lips and facial muscles make around the mouthpiece.
The embouchure allows the reed to vibrate, and the reed needs somepressure to do so. That means you need a firm embouchure, but not a tight one. Embouchures that are too tight can choke off the reed's vibration, resulting in a thin sound.
It's important to develop a firm and consistent embouchure because to make up for weak embouchures, many players bite on the reed to create the pressure needed and that will also result in a thin sound and poor intonation.
Steps to Create a Clarinet Embouchure
Step 1: Drop the jaw to the point that it is relaxed but not forced open. Move your jaw slightly from side to side to ensure it is relaxed.
Step 2: Roll the bottom lip slightly over the lower teeth, like a fitted sheet over a mattress.
Step 3: Firm the corners of the mouth and bring them forward and in.
Step 4: Place your index finger perpendicularly against the space or crack between the top teeth.
Step 5: With the upper lip, press downward onto the finger as if you were to push it downward off the front teeth, but do not roll the upper lip over the upper teeth. Note that your chin will automatically point downward in reaction to this.
Step 6: Remove the finger and, without changing anything else, insert about one fourth to one third of the mouthpiece into the mouth with the upper teeth resting on the mouthpiece.