Retention is a percentage of each payment (commonly three to five per cent) withheld by the paying party as security for the correction of defects, typically released half at practical completion and half at the end of the defects period.
In principle it protects the employer; in practice late and unreleased retention is a chronic cash-flow problem down the supply chain, and chasing it consumes commercial time long after jobs finish.
Clean records of completion, defects and their close-out shorten the argument: release turns on whether the conditions have been met, and evidence beats assertion.