Translation:It is cool in summer, and cold in winter.
There are different situations where we would change the い and put く, other times you will see くて before the comma. This will happen when there is more than one adjective and also before verbs (there are other places it will happen as well). : )
This く shows that the first thought/point is ending, so it is kind of like saying "and" without actually writing it.
These use く:
心もやさしく、うつくしい = (She) is kind and beautiful
いそがしく、話せない = (She) is busy and can't talk
These use くて:
さびしくて、泣きたい = I'm so sad, I want to cry
うるさくてごめん = Sorry for the noise