What is the relationship between the so-called ground and the positive and negative poles in the power supply
"ground" is a very important concept in any circuit. Because in any circuit, voltage is one of its most essential attributes.
In high school physics, we have studied the definition of voltage. The specific content of the definition of voltage is not our concern, we are concerned with the relativity of voltage: that is, voltage is a physical quantity that describes a physical property (actually the ability of an electric field force to do work) between two points. Although we often use sentences such as "how many volts is the voltage at a certain point", this does not mean that "a certain point" really has the attribute of voltage, which is just an omission. The complete statement is as follows: how many volts is the voltage at a certain point (relative to the reference point).
Let's take another look at the instruments that can be used to measure voltage, such as multimeters (oscilloscopes). There are red and black pens for multimeters. When measuring the voltage at a certain point in the circuit, we will use the tip of one of the red or black watch pens to touch that point. What about the other watch pen?
If the other watch pen is not connected anywhere, the multimeter will show 0V. At this time, 0V does not mean that the voltage at that point is 0V, it is actually the initial state of the multimeter reading. Only after another meter pen is connected to other points in the circuit, the reading displayed by the multimeter is the voltage value between "one point" and "other points".
It can be seen that this "other point" can actually be chosen at will. That being the case, why not choose the most convenient and global point as the reference point? The answer is yes, this is "land".