From Halopedia, the Halo wiki
Transport, also called Pelican Transport is a support ability for all UNSC leaders in Halo Wars and select UNSC leaders in Halo Wars 2 respectively. In both titles, Pelican Transport serves to allow the player to transport infantry and vehicle units across the map via Pelican dropship.
In Halo Wars, the ability is simply called Transport and can be accessed from the Spirit Of Fire menu. It targets a group of units, and calls down one or more Pelicans, depending on the size of the group, from the Spirit of Fire to transport them from one area to another, as long as the target location is within the player's Line Of Sight. The Pelicans are not usable in-game, but have their own health statistics and can be shot down during transport.
While the Covenant employ a similar system in the campaign, using Spirit dropships, this feature is unavailable in multiplayer for both A.I. and Covenant players. This is most likely due to the fact that the Covenant have Gravity Lifts which transport their units to their leader unit. None of the transports (Pelicans or otherwise) have any weapons capabilities, unlike in previous games.
Halo Wars 2
The ability reappears in Halo Wars 2 now renamed as Pelican Transport. Unlike the first game, Pelican transport is now a leader-specific power and only available for Sergeants Forge and Johnson. The ability performs similar to the first game, though the Pelicans now have limited range and can only place units onto areas not covered by the fog of war.