How to define a relative path?

I have heard about relative paths and I think that they are of good use.

But, I do not know how to define a relative path.

For instance, I have a PowerPoint presentation in which I use an audio (or video) during the slide show. I place these files (the presentation itself and the audio(/video)) in a folder. I would like to copy this folder to a removable drive so that I can use it in any system, but to achieve this, I should give the file name of the audio(/video) as relative path.

How can I achieve this?

Save your presentation first and then save linked files to the same place you placed your presentation(same folder on the same disk). Your links will continue to work when you move presentation with the linked files.
You should put your files in the same folder as the saved presentation BEFORE linking to them.