A test house is an independent laboratory that checks gambling software against a regulator's standards. In Great Britain, games and RNGs must be tested by a test house approved by the Gambling Commission before they can be offered.
Testing typically covers the RNG and how its numbers are mapped to outcomes, the maths model and its RTP, that the game behaves as its rules say, and compliance with the Remote Technical Standards, such as minimum spin time and the display of net position. Test houses often simulate very large numbers of rounds to confirm the published figures.
Passing leads to certification of that exact version. Any change to the game normally needs retesting. Operators are also expected to monitor live game performance against the certified figures.