The buffer simply passes through to its right output whatever input it receives on the left side. The truth table for a one-bit buffer is the following.
If the input is unspecified (i.e., floating), then the output will also be unspecified - unless the "Gate Output When Undefined" option is "Error for undefined inputs," in which case the output is an error. If the input is an error value, then the output will also be.
Buffers are the most useless of the gate components provided in Logisim; its presence in the Gates library is just as much a matter of completeness (a component for each possible one-input truth table) as it is a matter of providing useful functionality. Still, it can be occasionally useful to ensure that values propagate in only one direction along a wire.
Poke Tool Behavior
Text Tool Behavior