The Bokeh-effect is how blurred you can get the background in contrast to the main object.
In tests and reviews of lenses one often run into the term ‘Bokeh’ which is refered to when the aspects of a lens is to be explained. The word tells nothing about what it means and it is one of those words many amateurs just ignore beacause it sounds too complicated. The truth is that the ‘Bokeh’-effect is something that probably most photographers have faced, both planned and unplanned and can be used to create really good portraits and close-ups.
Look at the merged photo above taken by me at a beautiful flower. The upper left-part of the photo which was taken using an f-number of 22 have almost the entire area in focus and you can even notice objects in the background. This is not the case in the lower right-part of the photo which instead was taken with an f-number of 1.8 where the entire background is blurred onto a soft colorful cloud. In short the ‘Bokeh’-effect is how blurred you can get the background of a photo in contrast to the object which you have in focus.
Often you find lenses with great Bokeh to be fixed lenses with a low f-stop value. This effect can also be applied in post-production by adding selective blur to parts of the photo in a photo edit application like Adobe Photoshop.