An API (Application Programming Interface) is a way for systems to communicate with each other.
Cryptocurrency exchanges provide this feature as a means for other systems (such as Quadency) to connect and retrieve data as well as perform actions such as placing and canceling orders.
Quadency requires API keys for your exchange trading accounts so you can take advantage of the features available in the platform - including portfolio monitoring (which requires "read" permission) and trading (which require "trading" permission).
When you create API keys for an exchange account, be sure to grant only the permissions that you plan on using with Quadency. Do not re-use these keys anywhere else, or share them with anyone else. You also don't need to store them as you can create new ones if needed.