Reinforcing in Evony moves your troops from one town to another. This can be useful for defense or just to reallocate troops to a different town.
You can reinforce your allies towns by having them check the "Allows troops of allied members to garrison your current city" box in the Embassy (Only level 1 needed).
NOTE: Troops will use food of whatever city you put them in. Make sure you don't exceed your production in food.