"First mission" refers to the initial task or objective someone or a team is assigned to accomplish. It could be the first step towards achieving a larger goal or the starting point of a new endeavor. Full definition
The game still launches a tutorial to guide you through all the commands in the game, and in a matter of minutes you will be given your very first mission into the wild. (invisioncommunity.co.uk)
As you'll remember, the team's first mission doesn't exactly go as planned. (cinemablend.com)
Will he succeed on this, his way - important first mission? (tubitv.com)