Mods in Destiny 2 allow you to take advantage of additional perks. You can attach them to weapons and armor and enjoy the benefits of improvements to things like weapon handling, reload speeds, how much damage you can take, and how fast you can recover.
Mods can be earned in many different ways in the game, but some of the best mods are sold by Banshee-44, the Gunsmith in the Tower. Banshee can sell older mods that you cannot get from any other source, and some of them are the best mods in the game.
You can buy a random Weapon or Armor mod from Banshee-44 for two mod components and a thousand Glimmer, and to get mod components you will need to complete daily bounties for Banshee.
You can get four bounties from Banshee with every daily reset that will reward you with two mod components and one enhancement core. While you can buy more bounties with Glimmer, these will only reward you with experience until the next daily reset. So that means you can get eight mod components per day.
The mods that Banshee sells tend to cost 10 mod components, so it is best to do some bounties every day and build up a stock of mod components so you can jump on good offering whenever they appear in his shop.
Banshee will also give you two weekly challenges that can reward you with upgrade modules, and you can also do a total of eight daily bounties each week to earn a powerful engram from him, making him a great source of Power in the game.