[Script] Dialogue between two characters. (Level script?)

I'm making an area on the opening to Route 1 where two rocket grunts briefly talk to each other, then walk past the player into Pallet.

If I'm not mistaken, it has to do with the header tab in advance map.

Could someone sum up the explanation?

You're looking at either:
1) An S Tile script where you write the script and it triggers when the player walks on the script.
2) A level script, using the header tab, which triggers the moment the player walks into the map.

Your question is just the general new guy question, so a short answer is no, you can't sum up the explanation, go figure it out yourself.

Long answer, for things to work when you're ROM hacking you need to learn everything which takes some time, as we're glad to help, no one really wants to spoonfeed anyone information/make someone else's hack for them/etc.

I recommend you to read this tutorial, which is the best scripting tutorial around.
Even when you're a pretty experienced scripter like myself, I always keep a tab with this open whenever I'm hacking.
https://www.pokecommunity.com/showthread.php?t=164276

Also, regarding level scripts, there's a different good tutorial here.
Level Script Tutorial:
https://www.pokecommunity.com/showthread.php?t=191500

I'd honestly write more and explain, but that's really all the information I can feel good with handing over.
I'd rather see you fail multiple times and learn by yourself, then you get creative, think outside the box and more, rather than just straight up teach you the answer.

Good luck, if you're struggling feel free to contact me via PokeCommunity Discord, I go by the name Bliz But Better.

Also hack Emerald.

Got it.
Thx