Isospin definition
Isospin is a quantum number related to strong interaction. In 1932, Heisenberg introduced the isospin to explain the symmetry of the newly discovered neutrons. For particles with the same strength but different charges, it can be regarded as the same particles in different charge states. We use isospin to describe this state. Isospin is not a spin nor a unit of angular momentum. It is a dimensionless physical quantity. It is called "isospin" only because its mathematical description is very similar to spin.
In the process of strong interaction, the isospin is conserved, but in the process of weak interaction and electromagnetic interaction, the isospin is not necessarily conserved. The isospin of hadrons reflects the symmetry between the upper and lower quarks that make up hadrons.