'Boke' means 'stupid, unaware or careless'.
More commonly referenced by English speakers as 'bokeh', it refers to the clutter in the background of your image you forgot to think about while you were busy concentrating on your subject. Amateur photographers are often victims of excessive bokeh because it takes time to develop full awareness of everything that might be happening in the frame.
Photo-bombers take advantage of 'bokeh' to insert themselves inappropriately into your image - sometimes improving it. Here's Dave Chapelle 'doing a bokeh':
To avoid bokeh in your pictures, cultivate awareness of everything in shot - in Japanese, we call this 'Kaze-wo Atsumete;.
If you shoot at a wide enough aperture you can blur out unwanted background elements. This technique is called 'toneh'. For an explanation of 'toneh', see here: