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 an 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)