Can you help me understand what are independent causes

An independent clause is a clause that can stand by itself as a simple sentence. An independent clause contains a subject and a predicate and makes sense by itself. Independent clauses can be joined by using a semicolon or by using a comma followed by a coordinating conjunction.

Example:

I enjoy sitting by the fireplace and reading.

She wants to travel the world and see wonderful sights.

Our planets revolve around the sun.

The professor always comes to class fully prepared.

explanation:

it refers to the definition of full-contact sports, where the condition is that the sport must have athletes must "purposely collide with each other

what does the teacher mean