How do the pre-signals work in OpenTTD?

How do the pre-signals work in OpenTTD?

I'm reading their Signals article on their wiki, and it's still not 100%
clear to me.
I'm trying to (and failing) to understand the Entry signal, (and
subsequently, the Exit and Combo signals, because Exit "Works the same as
Entry but..." and Combo "Combines Entry and Exit").
So here's what it says on Entry signals:
Block Entry Pre-signal
An entry pre-signal prevents trains from entering the following block,
unless there is at least one green exit signal on the opposite side of the
block. This prevents trains from entering the block until there is an
available exit and obstructing it. Note that depots have a built-in two
way signal (it works as an entry pre-signal in a pre-signal set). If there
are no signals designated as exits on the block directly behind the entry
pre-signal, it behaves as a normal block signal. This is convenient for
bi-directional pre-signals where only one direction needs pre-signal
functionality.
Can anyone please provide a clearer explanation, and perhaps tell me how
do Exit and Combo relate to it?