PPI stands for “Pixel per Inch”. It describes the point or pixel density per inch measured at approx. 2.54 cm. The point density, also known as resolution, is an important measure for the level of detail in the reproduction of a rasterized images and one of the main quality aspects of technical reproduction processes.
The term DPI (dots per inch), which originally comes from the field of printing, is often used in this context. The DPI value describes how many dots a printer sets per inch. During the transition into the digital age and the spread of digital display devices (TV, monitor, smartphone display, etc.), the term was adapted to the pixels. Hence, both the sum of printer dots and pixels are described on 2.54 cm. Both values describe the image resolution and are used in the field of digitization to define the resolution with which an original should be scanned.
Usually round values are used, for example 150, 300, 400 and 600 ppi. However, it should be noted that a high resolution does not necessarily mean that you get a high quality image. The highest possible resolution is always recommended, but factors such as color rendering, depth of field, contrast display, possible distortion, etc. are also decisive. A good result always depends on numerous parameters. You can imagine that you use a camera once in automatic mode and then independently adjust various settings such as exposure time, aperture, white balance, etc. as you like. The resolution of the camera is always the same, but the images will differ extremely from each other.