Try Mode is a custom game whereby players can select and try any hero in a mock game. This is typically done to test abilities and/or when considering a hero purchase.
During Try Mode you get a mini version of the game where you play 2v1 against AI to try your chosen Hero. It always selects Malfurion as your partner and Arthas as the enemy by default. A little down the lane there is a Target Dummy and a Healing Dummy. Additionaly, there are some Mercenary Camps: Siege giants, Knights and a Grave Golem.
During this mode, players have access to a variety of options including the ability to select talents at will, to instantly complete quest talents, and to select any ally they choose to assist them. They can also opt to go it alone.