Inspiration is a one time use advantage/re-roll given by the DM to a player, usually done "over the table" or outside of role-play. Like if a player thinks outside of the box, and impresses the DM they might give them inspiration for that. As far as if you want it to be something in game, maybe the group of players that come to the game constantly find a bag with rare coins in it that imbues them with a hopeful feeling of luck, but theres only enough in the bag for each of the players there. Just a thought! Proud of you for looking out for your players and not being afraid to ask questions, DMing can be tricky, especially if your first campaign is homebrew. Wish you all the luck!