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.
Articles by others on the same topic
A genetically encodable protein that emits fluorescence after appropriate excitation. Fusion constructs can label cellular structures, but expression and illumination must be controlled for effects on the process being observed.