Mesons :

Mesons are unstable sub-atomic particles. They were discovered by Hideki Yukawa.
A meson consists of a quark and an anti-quark which are binded by strong interactive forces.
The size of a meson is about 1 femtometer. A meson is a part of hadron family (Made up of Quarks). The pauli's exclusion principle does not apply to the mesons as they are bosons.
Mesons participate in weak and strong attractions aswellas in electro-magnetic attractions.

Now, if we've got matter, we should have anti-matter.
That's where anti-meson comes into play.
A mesons got a corresponding antimeson where quarks are replaced by their anti-quarks and anti-quarks are replaced by their quarks.
A negative pi-meson or a pion is made up of an antiqark and one down quark but a the positive pi-meson is made up of one up quark and one down antiquark.Smaller mesons are more stable when compared to the bigger sized mesons. A meson is a binding particle. It binds Protons and Neutrons and keeps them in place. 

A meson binding proton and neutron