BusyBox is a free software released under the terms of the GNU General Public License that provides a number of Unix tools in a single executable file. Many of the tools BusyBox provides are designed to function with interfaces linked with the Linux kernel. BusyBox can execute and function in different operating system environments, namely Linux and Android. It is used by several operating systems executing on embedded systems. BusyBox was specifically designed for embedded operating systems, which have limited resources.
Techopedia explains BusyBox
BusyBox can be customized according to the requirements of the operating environment and can provide a subset of more than 200 utilities. The complete list of utilities that are implemented are available on the BusyBox website. BusyBox uses the ash shell and provides most of the utility tools found in the Single Unix Specification.
The single binary approach helps BusyBox to share the code between multiple applications without the need of a library. It also reduces the overhead needed for executable file formats.
Sharing of the common code among applications and size optimized routines allows BusyBox to be much smaller than a system with full versions of utilities.