They compliment how you look

A compliment about your appearance is cheap to give once and expensive to keep giving. The signal lives in the second and third one, never in the first.

How to read it

Appearance sits among the few things people deliberately avoid mentioning to someone they do not want to be misread by. That is why these compliments get withheld so often, and why repeating them is a decision rather than good manners.

Specific beats generic every time. Telling you that you look nice is politeness. Noticing something you changed means they were paying attention before you changed it.

When it means nothing

In some circles this is simply how people talk, and between old friends it can be pure warmth with nothing underneath. Shy interest also produces no compliments whatsoever, which is why this one should never be read on its own.

Your move

Take it properly when it comes. Deflecting a compliment teaches somebody not to risk a second one, and you need the second one to know what the first one meant.