The Year of Hell was the name given to the time period in which the starship Voyager entered into Krenim space.
In one timeline that it happened, Captain Janeway and B'elanna Torres were killed in the initial attack and Kes was still part of the crew. Though the ship suffered damage and the crew suffered losses, they managed to get through that year and Krenim space with the ship still in one piece.
In another timeline, where Seven of Nine is part of the crew instead of Kes, a Krenim-developed weapon ship was obliterating civilizations one race at a time, causing changes that resulted in Voyager suffering greater damage from Krenim warship attacks. Janeway put an end to this timeline by ramming Voyager into the weapon ship itself, destroying both vessels and creating a new timeline in which the Krenim pose no threat and Voyager simply goes around their space.