Stamina is a mechanic present in Celeste. It limits Madeline's ability to climb. Madeline, by default, has 110 stamina, and she always regains said amount when collecting a dash refill or landing on the ground. Holding still onto a wall, climbing upwards a wall and climb-jumping all use up stamina.
Madeline loses 10 stamina per second whilst holding a wall, 45.45 stamina per second climbing upwards, and 27.5 stamina per climbjump. When stamina reaches 20, Madeline will begin flashing red, and will no longer be able to hold a new wall (but can continue holding or climbing on a wall she is already holding onto or climbjump from said wall); when stamina reaches 0, Madeline will no longer be able to hold any wall. Madeline can still activate blocks from its side regardless of stamina; as such, she can still make Zippers work with 0 stamina.
Neutral jumps allow for bypassing this limit, by climbing without losing stamina.