You can lock matches against further changes by starting a phase.
For three reasons: it stops finished matches being changed by accident, it is required to generate the knockout after the group stage, and it makes clear to players and visitors which matches are final.
No. The scores of that phase are then locked and the system works the standings and the knockout through. So only start a phase once all its results are entered correctly.
On the Results tab or under Progress. Click Start phase next to the phase you want to close off.