This is the 347th most frequent Portuguese word.
principalmente
Mainly.
Here, 'principalmente' is used to highlight a specific example from a general category mentioned earlier (fruits).
Eu gosto de frutas, principalmente de maçãs.
I like fruits, especially apples.
In this sentence, 'principalmente' serves to emphasize that the condition (hot weather) is notably present during a specific time (summer).
O clima está quente principalmente durante o verão.
The weather is hot mainly during the summer.
The word 'principalmente' here is used as an adverb to specify a context (big cities) where the stated situation (traffic difficulty) is more prevalent.