When you move into a newly-built house it is possible that there will be defects and imperfections. A snag list records the defects that have not been completed properly and which you as the new homeowner have the right to have remedied by the builder or developer prior to moving in.
When you purchase a newly built property be aware that the building work will often have been performed by sub-contractors working under tight time-constraints and minor details may have been overlooked in the rush to have everything done before the planned completion date.
Drawing up the snagging list is often the responsibility of the buyer. Getting the faults dealt with before you move in will save you having to chase the builders at a later date.