Ciro Santilli You modify the DNA of a cell and stick a fluorescent protein right before or after another protein. Then when it gets translated, the GFP is stuck to the protein of interest, which hopefully hasn't lost its function as a result, then you can just see the protein of interest.
Using green fluorescent protein as a protein tag.
The 3D structure of GFP is so cool. It is so clearly a bottle with a fluorescent bit well isolated right in the middle. Like a little lamp.