A lot of people hear about the word open-source and figure that it's another esoteric term that software engineers coined to make themselves look fancy. These people are partly right, we software engineers like to sound fancy and intelligent. Believe me, as funny as it sounds, clients are averagely impressed by people who use fancy words. The assumption is "if I don't know the meaning of what he's saying it means he's really smart, at least smarter than I am in this field, which means he can do the job".
Humor aside, open-source is just a quick way of saying the source code of a digital infrastructure is made fully available to the public, of which said public (software engineers basically) can contribute to the code, redistribute the code and/or modify the code.
Usually, source code is an intellectual property that's guarded by organizations. However, organizations that want to grow quickly, get more developers using their products and expand the functionality of their digital infrastructure tend to go with open-source because it gets you all of the above without having to pay the developers (as they're voluntarily doing all of the above).
An easy way to remember this is that water (H2O) is open-source, Coca-Cola isn't.