a protein purification technique to “capture” your protein of interest based on something unique about it, such as an affinity tag you added to the end of the protein when you were doing the cloning. When you flow a mixture of proteins containing that unique-thing-having protein through a column of little beads (resin) that are specifically-sticky for that unique thing, your unique-thing-having protein will stick but others will flow through. Once you’ve washed all the non-stickers off, you can introduce a competitor like a unique-thing mimic, to push your protein off. A common form of affinity chromatography is a His-Tag and IMAC (Immobilized Metal Affinity Chromatography) such as a Ni-NTA column. Another affinity chromatography option is a strep tag and streptactin resin. More on both of those in the links below.