A wire segment equalizes the signal between two wire nodes,and a wire node equalizes the signal between several wire segments and component i/o.If they cannot equalize the signal(for example if two component outputs are connected and one of them is 164 and the second one is 17) a conflict will happen.In real life when a conflict happens there will be a) lots of current means lots of heat b) failure of attempt to be bistable(i.e converging onto a certain binary value,instead of somewhere in between),but in TC it just gives a Short Circuit error and stops the clock(because it is usually undesired unless you know how the transistors work and you are working with an analog circuit rather than a digital one)
A wire node/segment can carry a value or not(not carrying a value is also called Hi-Z where Z means impedance.When "this (bundle of) wire has a lot of impedance" is the most you can specify,it won't be carrying a value,otherwise you would be saying that value).When it is not carrying a value,other things can overwrite it.Wires start at Hi-Z state until connected.
Although in real life a bundle of wires can have some wires Hi-Z and some others tied to a certain potential(having value),it is undesired when the bundle of wires represents a number or anything that only represents one thing.So TC doesn't allow that.Additionally,TC makes all Hi-Z 0 if fed into/out of a component(for example,when inputting to most parts,or outputting from a non-switching output or a switched on switching output)