Let's see the full idea, which comes at the end of Coleridge's poem about a man possessed:
"...He prayeth well, who loveth well
Both man and bird and beast.
He prayeth best, who loveth best
All things both great and small;
For the dear God who loveth us,
He made and loveth all."
This is what the Ancient Mariner learned upon the sea; this is the wisdom he received from what he had done and what he had seen.
Remember, the Ancient Mariner thoughtlessly killed an albatross and it was hung around his neck by his fellow shipmates. After a long time of torment on the sea, and after everyone on board was lost except for him, the Mariner came to see a single, elemental truth: Thou shalt not kill. Moreover you should respect and love all of God's creatures as He does, no matter how big or how small, smart or ignorant, slimey or elegant. If you come to know and respect God, pray to him for your salvation and the salvation of all of life, then you will know the true meaning of love.
That's what those words mean... clear and simple.