After importing your raw diffraction data file into Match! (e.g. by pressing the '+' button in the upper right corner of the pattern graphics, or by using the "File/Import/Diffraction data" command in the menu), take a critical look at your data. Sometimes there are areas (typically at low 2theta values) that contain effects which have nothing to do with your sample, e.g. cutoff or primary beam radiation.
If this is true, you should remove these "surplus" area(s) ("trim" your diffraction pattern), in order to focus on the analysis of your sample’s diffraction pattern. You can do so by moving the mouse cursor to the left (or right) border of your diffraction pattern until it changes ( or ), press the left mouse button, and move the mouse to the right or left until the area you would like to remove has been marked. After releasing the mouse button, your diffraction data will be trimmed (i.e. the unwanted areas will be removed).