Hi, I have searched, but only found it's applications, so I was wondering if someone can please explain me what that is.

That is a very generic phrase. It essentially mean proteins being expressed at different levels. It can apply to all kind of different contexts, i.e. without further info it is virtually impossible to discuss it more.

Thanks for the reply, the context was something like why it is important to use a loading control when you are using a western blot in an experiment using differential protein expression. I assumed it was about using protein expressed with different concentrations or something like that, so looks like I wasn't so wrong.

Most likely it refers to the analysis of protein expression under different conditions. E.g. a cell line under normal conditions (whatever is defined as normal) and the same cell line exposed to some kind of stimulus.

Or a comparison of wild-type to mutant. The test then shows whether the protein(s) under investigation are expressed at different levels and are hence affected in their regulation by the stimulus/mutation.

Thank you