Computer peripherals are input- output- or storage- devices that add functions to the computer. Some of them are necessary, others are just bonus and adding functions to the computer.
Some of the really necessary input peripheral devices are the keyboard and the mouse. The most needed output device is the monitor, a other one that is often used is a printer. The so called storage devices are used for what it sounds like, as extern storage, such as hard drive or flash drive.
Technically everything that is not part of the core computer architecture (central processing unit CPU, power supply, motherboard and the computer case that has those three components) is a peripheral. There are external and internal Peripherals, internal peripherals are also called integrated peripherals. But typically when people refer to peripherals, they mean the external ones.