When you say for instance "Il y a une porte qui claque au premier étage." you mean that on the first floor a door is more or less regularly but constantly being banged against its door jamb by a draught. You use the same construction for other things, in particular windows and shutters, that are subjected to this action of the wind or of draughts.

• Il y avait des volets qui claquaient dans la rue et personne ne pensait à les arrimer au murs par les attaches prévues à cet effet.

There is an important matter of context which makes the difference between the meaning given to the verb in what precedes and a meaning very similar. That meaning of the verb is "to bang against something once or to close with a bang". It might be difficult to make out which is which.

• Elle se déplaçait maintenant dans la maison, passant d’étage en étage et ouvrant une chambre ou une salle de bain quand une porte claqua à l’étage supérieur. (Here a door banged once or got shut with a bang.)
• La porte a claqué, je l’ai entendue, il faut aller voir. (banged, got shut)
• La porte a claqué pendant un moment au rythme des coups de vents jusqu’à ce qu’il se lève et aille la fermer. (banged repeatedly)

The OP is asking about the usage of qui, qui is a relative pronoun (un pronom relatif) that replaces the word preceding it (l’antécédent) and usually acts as a subject. The sentence after qui is a subordonnée relative complément de l’antécédent meaning it complements that word. An English analogy would be who, this is the guy who loves you, who is replacing the guy here in the same way that qui replaced the door.