"Work not for the food which perishes, but for the food which abides unto eternal life, which the Son of Man will give you..."
This food is the life supply of God and when we eat it, we will no longer hunger or thirst! If we taste this food, the Bread of Life, we will be filled and energized/moved to seek after Him for more (John 6:26). (This is just like eating these super tasty pretzel chips from Trader Joe's...sooo goood....Once Susanne and I start, we can't stop! But really, this is literally the food that perishes.)
Since we shouldn't work for the food that perishes it seems like we should work for eternal life instead, but the clause after tells us that this life is given to us by the Son of Man! In a way, we need to buy this food (i.e. there is always a price to pay, Isaiah 55:1), yet we don't have to stir ourselves up and self-motivate to pursue Him, or work to earn our right to eternal life, or seek signs for proof of His reality--we simply need to open our mouths and eat!