UNRAID explain parity drive

[RobTi]

Hi so looking for a simple explanation of the above i have a truenas setup atm but find it too complicated to set up anything other than a plex share so looking at unraid. i have 4 x 4 Tb new drives and a couple of 120Gb ssd drives and a 1Tb old wd drive, but am lost at what a parity drive does and should i use one of the 4Tb for it or buy another larger drive ?
Thanks

 

[RobTi]

Watch some of Spaceinvader One's youtube channel, does some very good Unraid tutorials and explanations (some are a little out of date now but still helpful)

Thanks will do