On the lens you will see written something like f3.5-5.6. This is the aperture of the lens.
Aperture means a small opening in the lens through which light passes and evetually falls on the sensor creating the image. This size of this opening can be increased or decreased using the settings in camera. There is one diphragm with bladed that controls the size of the opening. Adiapragm is basicaly set of number of blades adjusted so that when combined form a hole.
If the opening is more that is to say if the aperture is more then more light will be allowed to pass through the lens and and less aperture will allow the lens to pass smaller amount of light. More light intake makes the photo brighter and in turn we get a better bokeh (background blur). For this reason large aperture lens are more preferred for outdoor portraits. Since the aperture is wide and lets in more amound of light in the same time compared to smaller apreture, a large aperture lens captures image in lesser time. For this reason high aperture lens are also known as FAST lenses.
Aperture can increased or decreased in some lens(18-55mm f 3.5-5.6) and its only one single aperture throughout the zoom in some other types of lenses (70-200mm f2.8). Aperture reduces in steps of 1/4. This is for the reason that we need to reduce the area of the opening by half.
Remaining part coming soon