OK. All the following is approximate, some terminology may be archaic, the mathematics may be imperfect but all of that is only relevant to pedantic morons, normal people will understand very well.
Here goes (and remember you asked for it!).
All airlines, (and bus/train/ship/etc. companies) sell tickets to travellers but a number of travellers just don’t turn up to use their reservation for whatever reason (meeting overran, taxi was late, overslept, …). Their seat is unused ie. wasted.
This happens to something like, say, 3% of all seats on all scheduled flights, and over an airline, over all airlines, over the industry, that’s a crippling financial burden that’ll bankrupt the industry. (Apparently we travellers don’t want to pay the higher prices to make up for it, but even if we did the airlines would still want to fill — ie. sell — the empty seats anyhow).
Note that at any moment across the world there are on average about 10,000 civilian aircraft actively flying at any moment including while you’re reading this, and most are scheduled flights.
That’s what no-shows are, and roughly the size of the problem.
What airlines do is to look at past load factors on a flight or route and if they’re regularly running at 90%, 95%, 98% then to save wasting a seat they’ll sell 10%, 5% 2% more seats in advance so that, after the no-shows, the plane will be exactly full, to maximise fuel efficiency, and profits.
Other flights/routes will suffer from the opposite problem which is “go-shows" who are people, for example with open (undated) tickets, who turn up, validly but unexpectedly, and want to travel. On those flights the airline will undersell tickets to ensure they can accommodate the go-shows.
As for overbooking (the rare occasions where 103% of ticket holders unexpectedly do actually show up) airlines are then in a genuine legal bind so they ask for volunteers at the gate and offer some really useful inducements to those who volunteer to he bumped to the next flight. (Inducements — bribes if you prefer — are things like cash, hotel accommodation, upgrades, extra free tickets to anywhere, several of these, and so on. As a volunteer to dig your airline out of the shit they’ve dumped themselves in you can ask for the the moon on a stick and probably get most of it, up to a limit). Those who need to travel right now take the flight, those who are flexible make a tidy profit, everyone’s happy. Some really smart professional travellers, eg. travel writers, actively look for flights likely to be overbooked so that they can profit big-time by volunteering to be bumped! Yes indeed, a useful trick to know ;-)
As for other refunds, that’s according to the terms of your ticket.
This applies to scheduled flights and airlines, charters and packaged holidays typically don’t have the problem of no-shows, and obviously go-shows are not possible on charters.