The phrase "left trigger" refers to a button on a video game controller that is usually located on the left side. Pressing this button can perform a specific action in the game, like shooting or aiming a gun. Full definition
While still holding left trigger and X, release the right trigger. (lifewire.com)
The control system to the game is like all racing game, right trigger for acceleration, left trigger for brake, and the analogue stick for steering. (ab-gaming.com)
Using the bow requires hitting left trigger to aim and right to fire. (theguardian.com)