Hi ukint,
Contacts or vias, these layers define cuts in the insulation layer that seperate conducting layers and allow the upper layer to contact down through the cut or contact hole.
Depending on aplication, the contacts may increase the resistance but in case of connectivity contacts reduce the resistance because more contacts means more path for the electron to flow from one layer to the other. Consider connecting a 20um metals, lets say for example metal1 and metal2 and you use a single contact of 5um, as compared for example when you use 4 contacts of the same size, the resistance of the latter is lesser compared to the first case.
Depending on the technology you do, a single big contact or more smaller contacts have both advanatage and disadvantage. We have a term called "perimeter to area ratio" which when you consider has an impact in terms of resistance.
Best regards,
Litz