Snapins in Powershell are like toolboxes, including many commands, specific to a tool, a software, etc..

The two most important (most famous) :

Microsoft.Exchange.Management.PowerShell.Admin

VMware.VimAutomation.Core

Here is the command to add a snapin :

Add-PSSnapin

A really easy way to add all installed snapins :

Get-PSSnapin -registered | Add-PSSnapin