okay, listen up: Neruda wrote under a pseudonym, his real first name was Neftali, a Biblical Hebrew name whose original holder was blessed by his father (Jacob) as "a deer set free"(That's in Gen 49:21). In the poem here the "deer [is given] to the roasting spit"; in this translation it's rendered as "young deer" but in the original Spanish it's literally "green deer." In America green connotes youth and inexperience, but as green is the color of death in Chile it's more likely tobe a punning way for Neruda to say, "deer of death".
Once you've seen that, "Mara Mori" is easier to understand. There's a common saying from old Latin: "momento mori." It means "remember you will die"; in modern bastardized Latin usage that reads as "souvenier of death"; "Mara" is a Hebrew name that means "bitter" (not necessarily in a bad way- bitter herbs, the Bible tells us, are good for you). So "Mara Mori" is "the bitterness of death".

You've got to use- not hide- your knowledge of death to really know the beauty of life, that's one cross-section of the multiple meanings of this poem