Q.my young peach tree
I had plastic sleeves on the trunk of my young peach tree so the voles wouldent eat the bark during the winter. I went to take the sleeve off and the inside was completely full of a thick jellylike substance. What is it?

It sounds like your tree has a canker. I have the exact same issue on one of my Peach Trees, it has not completely girdled but it is not the healthiest of my 2 Peach Trees.
It could be due to rubbing from the plastic sleeve. The bark of a Peach Tree is wounded easily and bacteria can enter those wounds.
This article will help you.
https://www.gardeningknowhow.com/plant-problems/disease/weeping-amber-in-trees.htm
Stay diligent on watering during dry spells, prune only in late winter or very early spring. Avoid any other wound to the tree, such as lawn mowers or trimmers. Follow a regular feeding schedule.