This will display the Bash help (manual) page for the specified built-in.
For example, help unset will show:
unset: unset [-f] [-v] [-n] [name ...] Unset values and attributes of shell variables and functions. For each NAME, remove the corresponding variable or function. Options: -f treat each NAME as a shell function -v treat each NAME as a shell variable -n treat each NAME as a name reference and unset the variable itself rather than the variable it references Without options, unset first tries to unset a variable, and if that fails, tries to unset a function. Some variables cannot be unset; also see `readonly'. Exit Status: Returns success unless an invalid option is given or a NAME is read-only.
To see a list of all built-ins with a short description, use