Missions in XCOM 2 are the Combat Scenarios that progress the main plot of the game. Completing missions advances the story of the game, and also earns Items which can be applied to Research Projects and Engineering Projects as well as other rewards such as Personal Combat Sims. Failing or completing the different missions presented in the game will move the Avatar Project gauge which is the alien's "win condition" closer or further from completion.
Before each mission, you customize the party loadout as well as the Soldier individual equipment loadouts to prepare best for the mission's objectives. A full party is 4 soldiers initially which can be upgraded to 6 via New Combat Tactics within the Guerrilla Tactics School.
Upon successfully completing a mission, you are presented with a mission summary screen that displays your turns taken, enemies killed, soldiers wounded and killed and an overall rating based on how you performed. You will then be presented with the Items looted from the mission. Failing to complete all the objectives within the parameters of the mission (such as turn limit, or rescuing x amount of civilians) or suffering a death of every soldier unit will result in a mission failure.
Both the Alien Hunters and Shen's Last Gift DLC packs contain a story mission. To access these missions, you must start a new game. Any in-progress games that access the DLC can access the new content like Weapons and Armor but will not be able to play the missions.
Supplies. Retaliations are scheduled and the counter that tracks the time until the next Retaliation strike can be seen in the window for Dark Events.