Editing is a process that is used across a range of practices, not simply photography. The editing job can be used for a variety of reasons but is primarily used to alter something.
Some of the reasons are
To alter something to remove imperfections
To enhance something
To alter something to change is meaning
To alter something to give it a specific meaning
To give something a specific narrative
Ultimately it is part of a preparation process to prepare something for public presentation.
The creator of an image, video, song etc. may not necessarily be the person making the final edits. Sometimes it could be a producer or magazine editor who may select the final images used for publication.
The edit could also be dependant on the platform that you are presenting it in. A magazine may have images along side text and will want to reflect a certain mood and this will have an effect on the images used. A gallery exhibition may be presented differently from photo book. From the moment of conception the final outcome for your work will be influenced by its end usage.
How to edit
It helps to be able to get some form of printed list of the images that you are to choose from; creating a contact sheet can do this. If the images are digital and you may have hundreds to choose from then it might also be good to reduce them before making contact sheets.
From a photography perspective, all artists will edit go through a process of redaction with their images, quite often narrowing the images down to one which through a combination of elements to leave the most relevant image.
The brief for my project requires me to produce 10 images but I will almost certainly make more than this and I will need to go through an editing process to select my final 10 images, I whilst editing I will also need to think about how I will present these when my project is complete.