A main clause (hoofdzin) is a clause that can stand alone as a complete sentence. It follows the V2 rule: the finite verb must be in the second position. Ik lees een boek. Morgen ga ik naar Amsterdam. These are main clauses. Coordinating conjunctions (en, maar, of, want) join two main clauses without disturbing either clause’s internal word order, so both remain main clauses.
A subordinate clause (bijzin) is introduced by a subordinating conjunction (omdat, dat, als, hoewel, terwijl, nadat, voordat, zodra, zodat) and has its finite verb at the end. It cannot stand alone as a sentence. Omdat hij ziek was is a subordinate clause — it requires a main clause to complete the meaning. When the subordinate clause comes first, the following main clause undergoes inversion.
A relative clause is a special type of subordinate clause that modifies a noun. It is introduced by die (for de-words and plurals) or dat (for het-words) and also has verb-final order: de man die hier woont, het boek dat ik lees. Relative clauses sit immediately after the noun they modify. Keeping track of which clause type you are in at any moment determines which word-order rules apply and is the key skill for navigating complex Dutch sentences.