high-side and low-side switching

There are two places to put a switch in series with a load: below it or above it. A low-side switch sits between the load and ground; the load's top connects to the supply, and the MOSFET connects the load's bottom to ground. A high-side switch sits between the supply and the load; the MOSFET connects the supply to the load's top, and the load's bottom goes to ground. Both interrupt the same current, but they are very different to drive.

Low-side is the easy, friendly case. With an N-channel part the source sits at ground, so the gate voltage you need is just a few volts above ground, easy for any logic or driver to deliver. The catch is that the load is never truly disconnected from the supply: its top stays live even when off, which can be a safety or fault concern (a short to ground downstream still energizes the load).

High-side is safer for the load (when off, the load is fully isolated from the supply) but harder to drive. An N-channel high-side switch has its source riding up near the supply when on, so to keep it on you must drive the gate ABOVE the supply rail, needing a bootstrap capacitor or charge pump. A P-channel part dodges this (pull its gate down toward ground to turn on) at the cost of higher on-resistance. Choosing the side, and the device, is a core decision in any power-switching or H-bridge design.