When you make an ability check or attack roll and fail, you can expend one use of Bardic Inspiration; roll the Bardic Inspiration die, and add the number rolled to the d20, potentially turning a failure into a success. On a failure, the Bardic Inspiration isn’t expended.
RollQuest ships the D&D 2024 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.