GoBo's (Go-between's) are devices that 'go-between' the light source and the subject. In some cases Gobo's can prevent light from Falling on the background, other times it can create a new dimension to a photo.
Gobo's can actually be anything from a spare piece of foam core, to a more elaborate Gobo projector. They can also take the form of a pasta strainer, patterns cut in cardboard, or cinefoil shaped like a snoot to restrict light from a constant light or strobe, or even a window blind next to a window. Gobo's can be placed just about anywhere on a set depending on what the photographer is trying to accomplish. One may even consider a grid a highly specialized gobo attached directed to a lightsource.
There are many reasons for using a gobo other than just restricting light. Sometimes a background looks just a little plain. A pattern can be projected onto a background to offer depth or even mystery to an image. Sometimes, photographers are not quiet lucky enough to have the sun in the exact position to create that stream of light. Other times the photographer may want to be creative and add the illusion that a large window frame is creating a shadow on the subject.
Sometimes though after a photoshoot, could stop and wonder 'what if' there was a light streaming across the background or subject. There are many Photoshop techniques for creating artificial light but some techniques can be somewhat complex and time consuming. One option to help with overall work flow may be to consider an overlay. Overlays come in different formats for photo editing but the most common including JPEG files and PNG files.
Overlays often come in a graphic shape like the one displayed here. To add an overlay to a photo the initial steps are the similar regardless of what file type you may use. In photoshop:
If you have a PNG file ( a file with a transparent background), the majority of your work is done. If you have and JPEG overlay, the next step is to choose a blending mode that works well for than image. Most commonly screen, overlay, or soft light work well but you may need to experiment to find the one that works best. If the overlay is too intense, you have the option to change the opacity of the overlay. You could also add a layer mask and remove portions of the overlay that you may not want in the image.
Here is also a tip of you happen to be using a projector or even a piece of foam core with a pattern. Take the image WITHOUT the gobo at first, then place the gobo where you wish it to be. Even the best plans don't always work out so by placing both images into photoshop, you can apply a layer mask and edit things that may not have worked out so well in the final image.
Give one of these techniques a shot, you may be surprised at how well it works!