the cord end is a umbilical cord with roots.
it implies that a new erdtree is made by literally birthing it through a female of fertile age.
a erdtree cutting is taken and possibly put inside them.
it connects to the mother through the "umbilical cord" part and to the ground with roots.

the "cutting gifted" tribe was gifted a cutting of the erdtree by marika (who else could gift cuttings of the erdtree?).
in the process they figured out the "sin of the erdtree"; could be the betrayal of marika in becoming a god, could be how the original erdtree was made.
in the end, for this they were exiled.

but they still kept going with their original plan.
they made clones of the erdtree using various fertile females.
they "tricked a god", they tricked the night into coming for this clones of the erdtree.
which is why every night 2, we fight under a white tree. through this tree, we fight a nightlord.
this means the trick worked; the nightlords have come to destroy this clone of the erdtree.
that's where the descendants of the cutting gifted come in and fight and kill the nightlords.

the ancestors gave themselves to this ritual; they are in the 8 coffins in the cript.
you can see roots on the ground; roots are how a tree takes its nutrients.
the ancestors "became nutrients" for these clones of the erdtree. for the roundtable itself.
they sacrificed themselves and their ancestors to save the erdtree and the lands between from the nightlord.