In Trail you play through a level forwards and then backwards from the point of your death.
When you are alive, the world is white and you drop a trail of memories behind you in the environment. Each memory you drop increases your score. If you touch a memory when alive it hurts you (sometimes it's painful to revisit the past).
The level is your life, so there does come a point where you will die - there is no escape, no-one is immortal.
When you die the world turns black and you start from the point of your death, or the last solid bit of ground in the case of falling to your death. In this state your 'health' which was reset on death constantly drains. I like to think of your health in this state being your attachment to this plane of existence. In addition to this, everything that could hurt you while you were living can still hurt you.
Collecting memories in this state boosts your health and increases your score. The memories, however, move away from you to make this just a bit harder. Your ultimate goal is to reach the first memory you left in the world.
Reaching this 'wins' the game, failure to reach it is judged an unfulfilled/incomplete life. To finish the game displays a short piece of text summing up your life, this is based on the score you achieved and your age (how far through the level you got).