Okay, imagine letters and numbers on your computer or phone. Different languages have different letters and numbers, right? For example, in English we have A, B, C, and so on. In Spanish, they have letters like Ñ and accents on other letters.
Now, imagine there are people who make different typefaces, like different styles for these letters and numbers. Some people make them fancy, some make them simple, and so on.
Open-source typefaces are typefaces that are made by people who share them and let others use them for free. This means anyone can use them without paying money to the person who made them.
Unicode is a way of organizing all the letters and numbers from different languages so they can all be used on computers and phones. So open-source Unicode typefaces are free styles of letters and numbers that can be used for any language that uses Unicode.
So when you're typing on your computer or phone, you can choose to use an open-source Unicode typeface instead of the standard one that came with your device. And you didn't have to pay any money for it!