Immunohistology, also known as "immunohistochemistry" is the use of antibodies to stain histological sections.
Historically, people would stain tissue sections with dyes to highlight different features (which is called "histology") - so there were stains that detected DNA, and would label the nuclei of cells in your sample. Or stains which detected connective tissue and could show tendon morphology, or articular (joint) cartilage, for example.
These stains were useful for a number of reasons, as they could allow you to see the shape and distribution of cells in your sample, so you might be able to see dead cells, for example, and help diagnose diseases: like too much connective tissue in the liver (indicitive of cirrhosis), or cell types not where they are suppoosed to be (cancer metastasis).
But they are limited - the development of new stains to detect other substances was pretty much random, as you'd just try a load of chemicals to see what worked, without being able to actually *design* a stain for the substance you were looking for.
And this is where immunohistochemistry comes in.
The "immuno" part just means that you use antibodies - and it is the power and specificity of antibodies which give immunohistochemistry its amazing flexibility.
In this, you isolate your biochemical substance of choice (maybe it's a new protein you are studying) and inject it into lab rabbits - just like vaccination. The rabbit forms an immune response to this foreign protein, and makes antibodies against it. After allowing the immune resonse to complete (which takes a few days), you extract some blood from the rabbit, and purify the antibodies from it. Obviously, there will be many, many different antibodies in this blood sample, but only one will recognise your protein - so you can use your protein to purify only *that* specific antibody!
You now have a purified sample of an antibody which will *only* recognise your protein-of-choice. And you can label this antibody with another substance (like a fluorescent dye, for example).
If you take your tissue section, and use this antibody as a "stain", you can therefore detect only your protein, and nothing else. And you can obtain more detailed information - like where in your cells the protein is found.