Most commonly a chasm with a bridge running across it, but could be any terrain that blocks character movement but allows ranged attacks (lava or even a controller-produced hazardous zone).
Initially I'm assuming you have no means of crossing outside of the choke-point (flying etc.).
If there's nothing to hide behind you're generally best taking the far end of the bridge/causeway/corridor, it gives you control over it and you can retreat if necessary.
If they have ranged and you don't, you need to get across the bridge or they're getting free hits that you just can't return and it's in their interest to stick a couple of defenders on the bridge to keep you occupied while the ranged attacks rain down on you. They're minimising the number of characters you're able to hit with and maximising the number they can hit you with.
If you have ranged and they don't, take the causeway and hold it. If you have a controller slap down a zone that'll slow them down and stand your defenders behind it so the enemy has to pass through the zone and then the defenders to get to you. If you're lucky the zone will give you the ability to slide or push the enemy off the causeway into the lava.
Of course if the enemy may escape to bring reinforcements you'll be wanting to get across the chasm and block the exits asap, which puts the imperative on you to get across the chasm and behind the enemy.
In the event that one or more of your characters can fly, the main advantage is that you no longer have to treat the bridge as a choke-point, can get around behind them to flank defenders or harass ranged attackers. If they can't fly it's even better as they can't get around behind you in the same way.
On a D&D-specific note, you may be able to use the Tenser's floating disc ritual to carry one or more people across, or even hold them 'safely' above the chasm while they use ranged attacks, though you'd have to prepare it in advance of the encounter.