What is a module?
In its simplest form a module is a subfolder in one of the folders listed in $env:PSModulePath with a PowerShell script of the same name (but with the extension .psm1 instead of .ps1): source
The psm1 file ends with an
Export-ModuleMember statement saying which functions and aliases to export., e.g.
Export-ModuleMember -Function Get-Foo, New-Bar, ... -Alias gf, ..