Side lobe artifact occurs when the beam of an off-axis side lobe encounters a structure and returns this off-axis object as coming from the main beam. This creates a duplicate structure on the screen but in a different area.
In the example below, it seems like there is a moving structure in the left atrium, but it is actually a side lobe artifact resulting from the mitral valve leaflet. This is important because oftentimes these side lobe artifacts may be mistaken for clots or foreign bodies. It is always a good habit to get multiple views to confirm that what you are seeing is artifact versus pathology.