I saw quite a few troll and joke answers to this question, which I find disappointing. So I will give you a proper explanation.

Quarks are a type of elementary particle, that make up the nucleus of an atom. What is an elementary particle? These are.

There are quite a bit of elementary particles. This table doesn’t even show all of the anti-particles though.

Quarks are point particles, meaning their size is so small that it is probably zero dimensional, or at least much smaller than the plank length.

But how do quarks make up the nucleus? Quarks usually pair up into groups of three. There are two types of these groups that are most often observed, one is called a Proton and the other a Neutron. That’s right, protons and neutrons are just groups of 3 quarks who’s combined charge equals +1 or neutral.

Also, quarks have half integer spin.

(Spin is a type of intrinsic angular momentum. It just exists we don’t know why or how, asking why or how is like asking why or how charge or mass exists. It just does for fundamental particles.)

The three quarks are always Up Quarks or Down Quarks, at least in the atom.

I do not want to further complicate things, but this image above, of the proton and neutron, isn’t the most realistic and is pretty misleading. The structure of a proton or neutron is actually more like this:

Lots of quarks popping in and out of existence randomly, virtual particles. But on average, there should be either 2 Up and 1 Down quark, for a proton, or 2 Down and 1 Up quark, for a neutron. Also, you may notice that there are “g” in the above image, those are another type of elementary particle called the gluon. Look at the top right of the elementary particle table. I won’t further complicate things but I hope this gave you a basic understanding of quarks.