Kant says (to the effect that) we must assume that there is an object that lies behind phenomenal objects/appearances (or however he puts it) to give them their meaning or sensible presence. This object cannot itself be represented by syntax (sensibility).
Likewise, we must assume that the syntax of Tractarian language has an object lying behind it that gives the syntactical elements their meaning. This object cannot be represented in Tractarian syntax.
Now, Kant does not want to provide us with unfamiliar items so he calls "things in themselves" "objects" - because we all know what an object is. Wittgenstein calls his "object" the "unsayable". We all know that there are some things that cannot be put into words.
Kant and Wittgenstein are transcendental idealists because they acknowledge that there are things afoot in the world that are responsible for things, but that these things afoot cannot be represented by the things they are responsible for. They are either "shown" (Wittgenstein), or made apparant to the understanding (Kant).