Learning curve
Also said as Skill curve, Onboarding slope
How quickly a player gets better at a game with practice. People say "steep" to mean hard to learn, though on a real chart a steep curve means learning fast.
Early progress is what keeps people playing. If a game takes hours to make any sense, most will not stay long enough to find out whether it is good.
Say it
“New players need one input in the first minute, two by minute five, and the full moveset by the end of the first hour. Don't show the crafting menu before then.”
Where you’ve seen it
Dwarf Fortress is known for one of the longest in games. Its community took "Losing is fun!" as a motto, because you will lose many times before you understand why.
Not to be confused with difficulty curve
A learning curve is about the player. A difficulty curve is about the game. A hard game can still be learned quickly.