The term multi-manager fund is usually used as a synonym for a fund of funds, although it occasionally has a somewhat broader meaning.
See fund of funds.
“Multi-manager” is a marketing term that obscures the nature of the product (its meaning is a lot less clear than that of “fund of funds”): it makes the resultant expansion of the fund range look less like mere reselling of bundles of other manager's products.
It also strengthens the perception (as credible) or the argument that by picking the best managers, a multi-manager fund/fund of funds is more likely to out-perform. This is, sadly, not supported by research: managers who have out-performed in the past are not statistically more likely to out-perform in the future.
It also allows a fund (even a new one) to claim a track record of hypothetical past performance, based on the past performance of the current managers. All this proves is that it is very easy to pick winners in retrospect.