A clause can contain one or more clauses within it. When a clause contains several clauses within it,  the containing clause is called proposition principale(main clause) while the contained one is called the proposition subordonnée (subordinate clause).

A subordinate clause, because it depends on the main clause, is seldom complete. To get its full meaning, the subordinate clause must rely on the main clause.

For example, in "Je voudrais que tu viennes demain",  the part in bold is a subordinate clause introduced by the relative pronoun que. It's a (subordinate) relative clause.

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Here is an example of a complex clause including a main clause and a subordinate clause. The subordinate clause is underlined for clarity sake:

Tom a mangé ce pain parce qu'il avait faim.  Tom ate this bread because he was hungry. 

Je reviendrai vous voir quand j'aurai du temps. I will come back to see you when I have some time. 

Elle ne comprend toujours pas les explications que vous avez données. She still does not understand the explanations that you gave.

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