An irreducible representation – often abbreviated irrep – is a representation that has no smaller non-trivial representations “sitting inside it”.
Given some algebraic structure, such as a group, equipped with a notion of (linear) representation, then an irreducible representation is a representation that has no nontrivial proper subobject in the category of all representations in question and yet which is not itself trivial either. In other words, an irrep is a simple object in the category of representation.
Notice that there is also the closely related but in general different notion of an indecomposable representation. Every irrep is indecomposable, but the converse may fail.
A representation that has proper nontrivial subrepresentations but can not be decomposed into a direct sum of such representations is an indecomposable representation but still reducible.