“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 exercises. Your friend asks you if it's been successful, and it has. You say:

It's made a noticeable difference.

make a noticeable difference

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.

Something "makes" a difference when it causes a situation to be different. The phrase "make a difference" is usually used for positive changes.