Certification is the sign-off from an approved test house that a particular version of a game, or an RNG, meets the regulator's technical requirements. In Great Britain a game must be certified against the Gambling Commission's standards before it can be offered.
Certification is version-specific. Each RTP version of a game is tested and certified, and later changes generally require retesting. The game served to you should match a certified build exactly.
It confirms the game is fair as designed: random outcomes, correct payouts, the stated RTP over the long run, and compliance with the game design rules. It does not mean the game is a good bet; every certified slot still has a house edge.