Muscles work by getting shorter. We say that they contract, and the process is called contraction. Muscles are attached to bones by strong tendons. When a muscle contracts, it pulls on the bone, and the bone can move if it is part of a joint.
Muscles can only pull and cannot push. This would be a problem if a joint were controlled by just one muscle. As soon as the muscle had contracted and pulled on a bone, that would be it, with no way to move the bone back again. This problem is solved by having muscles in pairs, called antagonistic muscles.
For example, your elbow joint has two muscles that move your forearm up or down. These are the biceps on the front of the upper arm and the triceps on the back of the upper arm:
Another example of a pair of antagonistic muscles is found in your legs. Here your quadriceps and hamstrings work together.
Muscles exert a force on bones when they contract. This happens for example when you lift or hold an object, or when you move a part of your body. If you hold an object weighing 10 N, keeping it still and with your forearm horizontal and your upper arm vertical, you would be exerting an upwards force of 10 N on the object. You could work out the force exerted by the biceps muscle to do this using the idea of moments. For more on moments see Physics - Forces - Moments. The way in which muscles and bones work together to exert forces is called biomechanics.