Hello Bernard,
Canonical SMILES is the SMILES molecule representation, that is unique for the molecule. It means, that you can use for creating molecule index as well as SMILES.
For example, we have a molecule CCN. It can be represented as CCN or NCC. But canonical SMILES always give the same result for both CCN and NCC.
Canonical SMILES computation procedure takes longer time than SMILES computation.
Best regards,
Mikhail