A specific stereochemical arrangement of amino acid residues in serine proteases that is characterisic of this class of proteases and that is responsible for their catalytic activity.

The residues are:

serine

The hydroxyl group of the serine side chain serves as the nucleophile that attacks the relatively electron-deficient carbonyl carbon of the peptide bond.

histidine

The histidine side chain is juxtaposed to the serine hydroxyl in such a way as to enhance its nucleophilicity.

aspartic acid.

The aspartate side chain is close to the imidazole group assisting its ability to act as a proton acceptor.

This is what it looks like, roughly (the imidazole ring is, naturally, pentameric - but it's easier to draw this way!)

                 ____
                /    \
Ser-O-H |||||  N      N -H ||||| O-Asp
                \____/