White label SEO is an SEO service provided by many SEO experts. It allows other marketing or web design companies to offer an SEO service, without having to do the work! Basically, the marketing company would ‘buy’ SEO services from an SEO expert and get them to do the SEO for marketing companies client, while putting their own branding and imagery on it, so it looks like it is done by marketing company.
This is an excellent way for a marketing company to expand the services they offer, as it means they can offer excellent SEO services for their own clients, but without having to learn SEO at all. They simply pay another company to do it for them, while charging their own client for the service.
Some people see this as a bit of an odd thing to do, as it is almost lying to your clients, however it is seen within the industry as a great way to offer a wider range of services. It also benefits the SEO companies, as it gets them more work without having to chase clients.