“It's made a noticeable difference.”
You are discussing your exercise routine with a friend who's also interested in exercise. You recently started doing a new type of weight lifting exercise. Your friend asks you if it's been successful. Positively, you respond like this.
It's made a noticeable difference.
Something "makes a difference" when it causes a situation to change. We usually use this to talk about positive changes:
Having an extra person around to help out has really made a difference.
Painting the living room really made a difference. It used to be so dark and depressing in there.
A "noticeable" difference is a change that's big enough for you to see and notice. It's not as big as a "big difference" but maybe a little bit bigger than a "slight" difference.