Basically, a computer is a set of components that are connected and interact with each other to properly function and offer the user an experience, through an operating system. Now, for these components and others known as peripherals to be recognized and interact with the OS, there is the need of a fundamental part of software called driver, allowing to communicate software and hardware properly.
In simple words, a driver is a software that allows the operating system to recognize and interact with a component or peripheral and execute one function or several of them. This interaction is carried out by an interface, and it is an essential part of every computer since it makes the hardware actually usable.
Without drivers, a motherboard would not recognize the storage solution attached to it; SSD, hard drive or any other where the OS is installed, so it would not be possible to boot whatsoever. A hard drive or SSD communicates with the motherboard thanks to a proper driver that establishes the interface and parameters to work and be recognized, by the motherboard and the later operating system to boot.
It is the same case with peripherals: a mouse or keyboard, flash drives or removable disks, gamepads, printers, a great set of receivers and emitters, or virtually any device that is connected to USB ports and other buses, all of them work through a proper driver that must be installed on the computer and operating system to be recognized and interacted by it and the user.
Fortunately, in many cases USB devices are plug and play, meaning that they already include the driver that allow them to work and interact with the OS, not requiring any prior installation. When this is not the case, peripherals include a CD or any other removable drives that store the driver.