Predictably, 'Show' is when you show the players something.You show them that they are in a village. You could create something that resembles the frontage of cottages and put a well in the middle.
You show them that they are being attacked by aliens - by having things that look like aliens attack them.
Showing guides the players in a more natural way than a clumsy tell. You could combine a tell and a show by having a filthy, half starved wounded child show up to tell them about the village being attacked (you still have to find a village for them to go to). It helps with the flow of the game.
Show has a lot more subtly.
You can 'Show' the players that the monsters attacking them here are the same as the monsters that attacked them last night by having the same tattoos on their faces. You could show that the noble who visits offering peace is somehow linked by having that symbol appear on the hilt of his dagger, which he plays with at the war meeting to discuss how to approach the threat.
You can also show players that they are in a cold barren area by putting them somewhere with cold blue light and smoke.
There are degrees of physrep. If you want to show the players that they are in a cave, but you only have a tent and some grass you'll need to work hard to make it look right. Do they need to see the outside of the tent? If so you probably want to obscure it's shape. Inside, can you hide the grass? Is there anywhere with a more cavelike floor? Is it a cave that's lived in? if so, could you use rugs to cover it, and again, drape cave coloured fabric from the walls.
If you need a river, you can have anything from a couple of ropes to show the boundaries (which may require you to tell them it's a river) to an actual river (via blue fabric, shiny blue fabric, and some high end fake river type effort).
You need to set the production values for your own game. We set our budget according to what we need. With SciFi games this was mostly a time sacrifice. We use scout camp sites a lot, and have dressed the entire inside of the buildings, and ended up accepting that the outsides of the buildings were much harder to dress. We've been known to build rooms inside of buildings to make them work for what we need. For one site we did absolutely minimal dressing. We rented a set of second world war tunnels and they didn't need it. We have events planned that we won't run until we find the right site.
If you haven't got it, and you can't build it, then there's no achievement in using it. If there is no cave, and you can't make a cave, then telling them it's a cave is probably not a win. Work out if you need a cave at all. It's not necessarily a cheap option, but if you're running several games and have storage, then you can reuse stuff in future games.
Having said that, I really want to run a game where everything is just identified by a label. People wear t-shirts with their character on the front. Tables have "control panel" stickers and a wall is labelled 'view screen'. Minimum phys repping and everything is a tell. I suspect it may not end up being the cheapest event we've ever run.
We're lucky here. We have the tools to phys rep a lot of stuff. We make things. This is where our skill sets really come into their own.