The resolution of an image corresponds to the number of pixels per inch it contains (1 inch = 2.54 centimetres) indicated by the term in DPI (dots per inch).

A pixel in computer graphics means each of the point elements that make up the representation of a digital raster image, e.g. on a display device or in a computer memory. In plain words, the pixel is the smallest element that makes up an image.

In images represented by computer data, the reproduced dots are usually so small and numerous that they cannot be distinguished with the naked eye, appearing merged into a single image when printed on paper or displayed on a monitor. Each pixel, which represents the smallest element of the image, is characterised by its position and values such as colour and intensity, which vary depending on the representation system adopted. (Source: Wikipedia)

Remember, lowering the resolution of an image e.g. from 300dpi to 200dpi decreases the grain quality of the photographic image and therefore sharpness.

A practical example: 15Mpx photo, approx. 3358x4467px

at 300dpi measuring 28.4×37.8cm

at 200dpi measuring 42.6×56.7cm