What's your favorite dish? Do you like pudding? My mum makes this delicious pudding that has chocolate chips in it. Every time I taste it, it has the right amount of sugar in it and the perfect amount of a very gingery flavor that makes a tickle on my tongue, always.
My mum tells me that this recipe was given to her by her grandma, and the original recipe didn't have chocolate chips in it and there was no cardamom either. Over the ages, as she acquired new tastes, she kept adding or subtracting things depending on her mood sometimes, sometimes mine. Isn't that a beautiful thing? Wouldn't it be amazing if you could cook a dish exactly like your mum did?
When you can look into how something was built and the source (or the recipe) is made available and can be modified and redistributed, it is called open source. If it is in the hardware domain, it is called open-source hardware, if it is in the software domain, it is called open-source software.