There are quite a few types of chokes on the market and several ways they work.
The most common after-market chokes work in one of 2 ways. Either
a) Standard constriction choke
b) Wad grabbing choke.
Standard constriction works by tightening the pattern of shot in the gun barrel over the last few inches causing the pellets to group together and pattern density to increase.
The more the restriction the tighter the pattern. Loosely speaking the following rules apply.
40,000 of 1” = Full choke
0 = Cylinder
Wad grabbers work by either using a mechanical restriction (a ring of studs grab the expanded base of the wad for a fraction of a second allowing shot to leave unhindered by gas or wad) or gas restriction to slow the wads down by a fraction of a second in the shape of channels similar to rifling in a barrel. As the wads reach the required area in the choke. The channels fill with gas and slow the wads allowing again the shot to leave unhindered by gas and wad. This then causes a shorter denser pattern giving greater pattern penetration at greater distance.
Remember pattern test your gun and ensure you shoot with its and your abilities.