Non- technical explanation - it means that a young shoot has been 'attached' to an older root to shorten the time for the plant to come into flowering condition.
Other experts here will be able to explain the process in detail. Grafting is done for a variety of reasons. Roses and fruit trees are other candidates for this process.

More likely a seedling with a branch from a mature flowering tree grafted on. That sometimes reduces the time from start to bloom from 10 years to 2 years.