The strong nuclear force is an attractive force that binds what?
It binds the protons and neutrons in the nucleus.
An atom consists of 1 positively charged nucleus and 1 or more negatively charged electrons cycling around that nucleus. In this atom there are 3 types of particles:
protons -> protons are pretty heavy with a mass of 1 u. Every single proton has a charge of 1+. Protons stick together in the nucleus. Most chemical reactions do not interfere with the nucleus. The amount of protons is also called the Atomic Number.
electrons -> electrons are very small compared to protons and neutrons. Electrons do not stick in the nucleus but circle around it. Every electron has a negative charge of 1- . The mass of the electrons is so small that we do not even take that into consideration by the total mass.
neutrons -> neutrons have approximately the same mass as protons ~1u and also stick together in the compact nucleus as the protons. The difference is that neutrons do not have any charge.
Now let's look at your question. We know that 2 positively charged particles repel each other. But if all the protons (1+) are together in the nucleus and are pretty close to each other they should repel right??
But that doesn't happen due to the nuclear force. This force keeps the protons and neutrons close together in the compact nucleus! And that is important for the stability of the atom.