What Termux actually is
Termux is a terminal emulator for Android that gives you a real Linux environment — its own package manager, its own filesystem, no root required. It's not a VM and it's not emulating another OS on top of Android; it runs natively on your phone's actual processor, which is why it stays fast even on modest hardware.
By default it's sandboxed to its own app storage, same as any other Android app. You can grant it access to shared storage if you need to read or write files outside that sandbox.
What people actually use it for
The honest list: running Python, Node, or Go scripts on a phone when a laptop isn't around; learning Linux commands without installing anything on a real machine; SSH-ing into a server from your pocket; lightweight automation and scripting.
It also ships packages like nmap and other networking tools, which is why it shows up a lot in security-adjacent searches. Those are legitimate tools for testing infrastructure you own or are authorized to test — same as on any other platform, using them against systems you don't have permission for isn't something this covers.
Getting started in five minutes
Install from F-Droid rather than the Play Store — the Play Store build has been stuck on an old release for a while and a lot of packages won't install cleanly against it.
Once it's open, the first two commands are always the same: pkg update && pkg upgrade, to sync the package index and pull the latest versions. From there, pkg install python (or node, or golang, or whatever you're after) gets you a working interpreter in under a minute.