It’s called a ‘dialogue tag’ and it simply means that you mix together the description of the scene with the actions/speeches of the character you wish to pinpoint.
Describe where they are or what they are doing, whichever is appropriate, in a sentence or two, then segue right into the dialog or action in the next sentence in the same paragraph.
Since you have just described either the character or some action involving the character, when you continue with dialogue or action your reader will automatically know who you is doing it.
This moves your action along much faster than if you divided your writing into description and also dialog/action because, this way, both are intertwined.
One of the easiest ways accomplishes more than just indicating which character is speaking or doing something.