The Black Simmer

So at first, mods and stuff for sims 4 can be a little complicated to grasp, but it gets almost effortless when you first start downloading them.
First,

retextures are just meshes that have the colour/design changed, for example, a black hair being retextured to be bright red. Meshes, are the objects themselves, and they can be hair, shirts, furniture, etc. but for most retextures to work you need the mesh.
You can't download most retextures without the mesh or it wont show up in game.

 Mods, are mostly just additions to the game made by fans, and you can find a lot on modthesims. To install them, and other custom content, you need an unzipper program like WinZip, and drop them in the mods folder that's automatically made when you download the game. its in Documents< Electronic Arts < The Sims 4 < Mods. If its not there, just make a folder and name it "Mods", no quotations, no embellishments, just the word Mods. If you change the name of the main folder, the game wont read it.
 I suggest making subfolders for your hair, skin, eyes, script mods, etc. before you have thousands of files in your game like I do. Its hell trying to find the one file that's messing up your game in a sea of 30,000 mods

You can get mods on ModtheSims, Tumblr, TheSimsResource, TheBlackSimmer, and a lot of people's personal favorite, Lana's CC finds, where they have a huge catalog of hairs, accessories, skins and everything else tagged from gender, life state, theme, and game. I get the majority of my cc from Lana and tumblr.

 You select your folder, and unzip it through winzip, I, personally have my computer set to where zipped folders/archives open automatically with winzip, but you can pick and chose if you want to do that or not.

You don't *need* mods, but they're nice if you want to add a bit of spice to your game with stuff like gameplay mods like get to school, path to fame etc. and you can also use custom content to add personality to your sims with hair, eyes, makeup and a lot more. They're definitely optional, but a lot of people like to use them to customize their game and pick up the slack that EA kinda left behind.

One mod I think you should get, just for convenience reasons, is MC Command Center, which helps you edit the game when it crashes. You can use it to kill sims, change outfits, and edit lots, etc. with the click of a button. But, when a big update comes out (expansion pack usually) it breaks MCCC and you have to redownload it, but that's one of the only drawbacks.

My Favourite Mods:
MC Command
Create a Sim Columns
Path to Fame Mod
Custom Traits

I hope I could help, have fun!