A white label is a casino brand operated on another company's platform, usually under that company's licence. The brand owner handles marketing and look; the platform provider supplies the games, payments, customer accounts and compliance, and holds the operating licence.
Many online casinos share a platform this way, which is why you sometimes see several brands with identical layouts, terms and game libraries. Brands on the same platform may also share customer accounts or self-exclusion checks.
In Great Britain, the licensee is accountable to the Gambling Commission for everything done under its licence, including by the white-label partner. The site footer should name the licence holder and its account number, which is the name to check on the Commission's public register.