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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:

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

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