“Backpressure” is a fairly common term regarding the resistance to flow of some piping system (ex: exhaust pipe) when the pressure at some point downstream of this resistance is known. In the case of an exhaust pipe, the outlet of the pipe is open to atmosphere, so we know the pressure at this point is atmospheric pressure. But because there is flow through the pipe, which represents a resistance to flow, the pressure is not atmospheric pressure all the way up the pipe to the engine. If you were to measure the pressure at various points along the exhaust pipe, you’d find that the closer you got to the engine, the higher the pressure would be. This is simply because there’s a resistance to flow of any fluid through a pipe. So the term “backpressure” simply says that there is a pressure drop through a pipe and the pressure at the inlet of the pipe is higher than the outlet and we generally acknowledge that it’s the outlet of the pipe who’s pressure is knowable. This is true for the exhaust on a 2 stroke or a 4 stroke engine. Clear as mud so far?
In the case of the 4 stroke, the desire is to eliminate ALL the burned gasses from the cylinder when the piston gets to TDC - so the more backpressure, the higher the pressure will be inside the cylinder, and the less exhaust gas we will be able to expel. We don’t want those gasses inside the cylinder, so we want the backpressure on a 4 stroke to be as low as possible. Here’s a decent discussion on 4 strokes:
In the case of a 2 stroke, the thing that’s clearing the exhaust from the cylinder isn’t the piston going up to TDC because the piston is in fact only a little past BDC when the exhaust gasses are displaced. For a 2 stroke, it’s the sweeping of the cylinder by the fresh charge of air and fuel which is pushing the exhaust gas out. If there’s no back pressure, the fresh air and fuel will not only sweep out the exhaust gasses, but they will also begin to exit with the exhaust gas, resulting in a loss of fuel and air which reduces efficiency. If the back pressure is too high, the exhaust gasses can’t all be removed by the fresh charge. So for 2 strokes, there is an optimal back pressure to produce the most efficient fuel use while maximizing power. This optimal back pressure must be tuned to a certain RPM, because above and below that RPM, one of the two problems will surface. 2 strokes have a very dynamic gas flow which is difficult to optimize because of this. Most 2 stroke mfg’s simply tune the engine at a given RPM and throttle setting which results in the best overall performance.
This is a rather simplified explanation of the 2 stroke, there are other considerations. A small amount of back pressure for example can increase the total air/fuel charge in the cylinder by raising the pressure at the point the exhaust port is closed off. If you’d like to read more, I’d suggest these: