Patching your client means that you download a patch file and you add it to your World of Warcraft installation folder inside the Data folder.
This Data folder contains all the visual assets needed for you to actually "see" the world when you login in the game.
There should be quite a few patches already in the Data folder and they are from Blizzard content up to Wrath of the Lich King.
Adding patches don't modify your cliet specifically but they give instructions to your client to display new maps, new visual skins and such when you login into the world.
If you start having issues with starting your game or otherwise, you can simply delete any extra patches you added to this folder.
If you have the space, it's recommended you always backup your Data folder and perhaps even the whole installation folder of the game before you do any client manipulation.