APS (Ammonium Persulfate) is a chemical used as an initiator in the polymerization of acrylamide gels for SDS-PAGE. It, along with TEMED, helps to initiate the crosslinking reaction between acrylamide and bisacrylamide, forming the gel. APS decomposes into free radicals, which start the polymerization process, allowing the gel to solidify and be used for separating proteins by size.