The first fact you should know is that forms or kata is not fighting.
The first form - Sil Nim Tao teaches about your centerline. Each move is done on both sides of the body because Wing Chun uses both sides of the body to engage in combat. It is the most important form.
The stance is NOT for fighting. It's a neutral stance. Sil Nim Tao may looks ridiculously simple (Wing Chun is meant to be simple), yet it can be extremely hard to understand. That is why you ask your teacher "What is this move for?" If the teacher cannot answer such a question, he/she is bad teacher.
The hard part is doing the form PERFECTLY! From the every beginning, your knees should align with the toes, but not bent any further or you'll hurt your knees.
When you do each move, you focus entirely on your ELBOW! It's always the elbow. The elbow sinking and inward (Don't exaggerate),
Shoulders pinned down (So if your shoulders hurt, then you did it wrong)!
Align your head as if it's being pulled up by a string.
Normally, your spine has a natural "S" curve, but if you do it correctly, your spine should be straight.
And a bunch of little minor details......etc
The second form is Chum Kiu - it teaches you how to take your centerline and move it. You learn how to shift, you lean how to take a step forward, you learn how to kick, how to contact your opponent (Building the bridge) and you learn how to use both hands together.
Your body should not lean forward or back, as if you're trying to perform it with a glass of water on your head. Your body should not bounce up and down when shifting. It should be smooth.
The third form - Biu Jee - it teaches you emergency moves if you screw up, it introduces new techniques such as thrusts to the eyes/throat, and elbow strikes.
Before Biu Jee, you're often taught the wooden dummy form, where you practice on a solid object, getting the correct angle and introduce new moves.
The main point of forms is that it allows you to practice alone of all the moves that Wing Chun has to offer and constantly reminds you of the basic concepts and principles such as Shoulders/elbows sinks, don't lean forward or back, centerline.
You can test if your structure is correct by having a partner push you as you try to take a step forward.
But forms are not everything, Wing Chun has drills, Chi Sau, sparring, weapons.
But forms are completely useless if you are clueless of what the movement is even for. Therefore, you ask your teacher "What is this move for?"