This is the 13222nd most frequent Russian word.
многоквартирных
"Of multi-apartment" or "multi-apartment (adj.)".
The word 'многоквартирных' is used to describe a feature of 'houses', indicating that they contain multiple apartments.
Этот район состоит из многоквартирных домов.
This area consists of apartment buildings.
Here 'многоквартирных' describes 'complexes', specifying that these complexes comprise multiple apartments.
Строительство многоквартирных комплексов продолжалось два года.
The construction of apartment complexes lasted for two years.
In this context, 'многоквартирных' qualifies 'buildings', referring to structures that house several apartments.