This is the 233rd most frequent Serbian word.
касније
"Later."
Here 'касније' is functioning as an adverb of time, indicating when the action of going to the store happened.
Касније сам отишао у продавницу.
Later, I went to the store.
In this sentence, 'касније' serves as an adverb modifying the phrase to specify a period within his career.
Био је све бољи играч касније у својој каријери.
He became a better player later in his career.
'Касније' in this example is again used as an adverb specifying the timing of the agreement to meet.