No-entry signals are an extension in JGR's patch pack. They completely forbid trains from continuing beyond the signal.
No-entry signals are a specific form of path signals, so they can be built in exactly the same way from the signal building menu. There is no way to build them in both directions on the same tile (it would also mean that this piece of track cannot be used in any direction, so why is that track there in the first place?). There are variants as semaphores and light signals, although they do not differ at all, not in their appearance nor in their behavior - at least when not using a dedicated signalling set.
Behavior
No-entry signals can only be passed when approaching them from the back side. When they are facing the train, the train cannot pass.
On a technical level there are some more differences to other signals: As no-entry signals can never be passed from the front side, they are a dead end for pathfinding - no path can be reserved going across that signal. Other signals are no such dead ends, as eventually they will show "Go!" and let trains pass, so that the pathfinder can determine the rails behind the signal as accessible. That also means that no train will ever run up onto a no-entry signal and then wait for it to magically allow it to continue its travel.
Usage
No-entry signals are useful in all places where trains shall be prevented from going a certain direction, but where a one-way signal cannot be placed. In this example the no-entry signal next to the depot blocks trains coming out of the depot to go back into the junction area to the left. If one would place a one-way path signal there instead, trains would stop at that signal and block the junction.
Lets assume that trains will want to reverse their direction at this station, because their next stop is in the direction they came from. This shall not be allowed here. You cannot simply put a one-way path signal in front of the platforms, as then the next oncoming train might stop right on the junction tiles when it is unable to enter the station, blocking other trains from leaving the station. Trains going to the station need to be able to reserve a path right to the platform, otherwise they must not even start going towards the station.
The solution is the no-entry signal, as used on the upper platform. A train coming in from the right will always reserve the path onto the platform, passing the no-entry signal from the back side. Once at the platform, it can only continue onward to the left. The lower platform does not have a no-entry signal, here a train could reverse directions - and would then head out of the platform without passing a signal controlling access to the track on the left (this works in game, it just looks wrong).