This is the 748th most frequent Croatian word.
ljeto
Summer.
The word 'ljeto' is used here as a noun to describe the season 'summer'.
Ljeto je najtoplije godišnje doba.
Summer is the warmest season.
Here, 'ljeto' refers to the time of year, expressing anticipation for the season.
Jedva čekam ljeto da odem na more.
I can't wait for summer to go to the sea.
In this context, 'ljetu' is the locative case of 'ljeto', used to denote the time when the action takes place.