What the use of toolchain?

Hello,maybe the most dumb question here, just curious what is "toolchain" tarball in every target device ?
i see "imagebuilder" to build a firmware, "sdk" to build packages, and what is the toolchain for ?
is it something i can extract to imagebuilder, or sdk directory, or in a build root and makes some advantage ?

Toolchain is the compiler/linker/other support utilities needed to take source code and create an executable or library from it - nothing can be built without it

In case you would like a pointer to the official documentation, here is a link: https://openwrt.org/docs/guide-developer/start

From what i read in official documentation, the guide is only using it in buildroot / build system , can i use it in image builder or sdk only ?
I mean if there are undocumented functionality.

The SDK is basically toolchain + parts of the OpenWrt buildsystem to process OpenWrt specific makefiles in order to produce .ipk packages.

The bare toolchain lacks these additional components and just contains compiler and linker executables, enough to cross compile and link C and C++ code for the target architecture but no further packaging utilities.

Essentially, it is a developer tool.