/bin is the directory for essential user-facing command binaries that must be available before any other filesystem is mounted. It contains the tools needed to boot the system, enter single-user mode, and recover from a broken install: sh, ls, cp, mv, cat, mkdir, rm, mount, and a few dozen others. The constraint is that /bin lives on the root filesystem, so it is accessible even if /usr or /home are on separate partitions that have not yet been mounted.

/bin/sh       /bin/bash     /bin/ls       /bin/cp
/bin/mv       /bin/rm       /bin/cat      /bin/mkdir
/bin/mount    /bin/umount   /bin/grep     /bin/sed

On most modern Linux distributions following the merged /usr scheme, /bin is a symbolic link to /usr/bin. The distinction between what belongs in /bin versus /usr/bin was historically about what was needed before /usr could be mounted from a network share; that constraint rarely applies today. The symlink preserves compatibility with scripts that hardcode /bin/sh or /bin/ls while consolidating all binaries under /usr.