Canonical Transaction Ordering is a different way to organise transactions in a block (different from Topological Transaction Ordering, currently used).
Bitcoin ABC's argument for CTOR
1. CTOR is simpler to implement than TTOR.
2. About 70% of block template creation time is due to ordering requirements. Eliminating this will improve performance.
3. Improves parallelisation
4. Easier blocks transmission
5. Security patch (large block with a transaction ordering that is as slow to validate as possible)
nChain's argument against CTOR
1. Unsecure (no technical explanation)
Note: nChain planned to implement CTOR in the past: source
It does require a hard fork.