We can use words or terms such as "and, if, or, as though, so that" in a sentence that contains two separate clauses in order to connect them. This is called a conjunction.
In the example given, the word "but" is acting as the conjunction. It is connecting the first clause which reads "Some children play on the swings", with the second that states "others went down the slide".
Most of the time it is best to use a comma directly before the conjunction to make the connection more clear, specially when we're dealing with independent clauses as seen in this example.
Hope this helps!
