Seasons in Diablo 2 is an optional game mode that lasts for a couple of months and forces the players to start a new character in order to participate. Each Season players will race to reach Lvl 99 or the highest lvl possible. Seasons also makes for a fairer play, as all players participating have to start from the start.
Players participating on a Season will only be able to play and interact with other players participating on the same Season. Players won't be able to trade or receive help from characters outside the Season. Once the season ends, the character will be available to play in the normal mode and a new character will have to be created in order to play the new season.