There are a few ways you can send encrypted information, but they all boil down to the same concept: Create a problem that is very difficult to solve unless you already know part of the answer. For example, I have a number that when divided by 7(a prime key in this case) has a remainder of 3. It could be 10, 17, 24, etc.... We then take that number's remainder, raise it to a power and multipy it by a large number to create a huge result. I send this result to the other party without sending my prime, along with the number I multiplied. They choose their own prime and do some more math with my result and send back the result. Since I know my prime, its simple for me to get the result but anyone watching would have to do a lot of math to get the answer.

Two excellent videos on the subject: https://youtu.be/NmM9HA2MQGI https://youtu.be/Yjrfm_oRO0w