Proteins are polymers. The monomers that make them are called amino acids. There are about twenty different amino acids, and a protein is made by joining hundreds of them in a long chain. This joining is synthesis. The chain then folds into a particular shape.
The order of the amino acids decides the shape of the protein, and the shape decides its job. Proteins make enzymes, antibodies, hormones, haemoglobin and the structure of muscle.
Proteins in food are too big to be absorbed. Protease enzymes in the gut break them down into amino acids, which are small enough to cross into the blood. Cells then use these amino acids to build the proteins the body needs. To test for protein, add biuret reagent. It turns from blue to purple if protein is present.