The offside rule gets far more attention than it deserves and in my opinion it's happened due to the increased popularity and quality of live televised football over the last ~30 years.

To me the best place to start is to understand what the rule is trying to accomplish. It's designed to discourage a player hanging out by their opponents goal the entire game in the hopes that somehow their team can somehow get the ball to them and they'll have only the goalkeeper to beat (or whatever poor defender was tasked with tracking them) - "poaching" we'd call it in 5 or 7-aside games where there is no offside rule.

How do you do that? Well you just try to make a rule that says "if you're inside your opponents half your teammates can't pass you the ball unless there's at least two players between you and the opponents goal line".

It only becomes tricky to explain or understand because:

- it is encoded within the laws of the game in a very specific way that's a little tricky to digest in its entirety

- most people know some bits of it but only half-remember little details and try to explain all of that at once and confuse both themselves and the person they're explaining it to

- some people think that the ball needs to be passed in a given direction, leading to (rare) situations where they're unable to explain an offside call or to incorrectly describe it as wrong

- some try to explain it in terms of one attacking player being beyond one defender, forgetting that it's actually two players on the defending side, one of whom is usually-but-not-always the goalkeeper

- people try to include exceptions when they explain it (you can't be offside from a throw-in, for example)

- local football associations try to tweak it to be more fair or to appease fans (but inevitably make it more complex and piss off fans). They might change it so you're offside even if you didn't personally receive the ball for example.

- it is often explained to or by someone who has had a couple of beers and is attempting to demonstrate it by moving pint glasses or beermats around on a pub table

- if you want to be technically correct, it's not just "is a player in an offside position" it is "is any part of an attacking player's body that is allowed to play the ball offside" (i.e. you can technically be offside of your head, legs, chest, ass are offside .. but if it's only part of your arm that's in a given position you're not offside)

- etc

Each added detail or half-remembered corner-case can make any explanation more convoluted and hard to understand.

But for 99% of situations the sentence "if you're inside your opponents half your teammates can't pass you the ball unless there's at least two players between you and the opponents goal line" will suffice.

If you want to simplify it as "... at least one defender ..." that's fine in most cases. A player on the attack who is trying to avoid being offside will just look out for the last defender, assuming the goalkeeper will be stood near their goal. A player who is attacking will play the ball assuming their teammates were onside unless they're obviously not. A player who is defending will keep an eye on the furthest-back defender - there will be a loud member of the defense coordinating this, telling them to move up or down the field.

If you want to think about it in more detail, remember what parts of the body the attacker is allowed to touch the ball with.

If you're watching a match in person, you likely won't be in a position to make call it in any more detail than that anyway.

If you're watching on TV the commentators will break an offside call down in excruciating detail for you and show you lots of stupid graphics and slow-mo replays.