When people refer to toning, they usually mean “to look toned”. For most, toning means a firm defined body or body part like the arms, not bulky and not flabby.
Toning comes in 2 parts. First of all you must work the muscle through exercising, to make it very slightly bigger, make it denser – for firmness and make it more contracted – to look good. The second part of toning you need to understand, is that if you have body fat over the toned muscle, you will never see the muscle. Lots of people have strong, toned 6-pack abs, they’re just not visible because they’re buried beneath body fat on the belly.
Doing a million sit-ups might tone your abdominal muscles, but if you have body fat on top of those lovely abs, you will never see them. Doing sit ups 24 hours a day 7 days a week, will not get you visibly toned abs and it is a complete myth that you can turn fat into muscle. The muscle must be toned through exercising and the body fat must be reduced mostly through diet, so you can then see the toned muscle.
Having a proper diet AND exercising is the only way to achieve real visible toning. You cannot do one, without the other unfortunately. So if you want to tone your arms, doing lots of tricep exercises may tone the muscle but if you still have a bad diet and have too much body fat, then you will never see that muscle tone unfortunately.