Not sure how much programming you know, but we use special languages to express things so that computers can understand (you might have heard of C, JavaScript, Python, etc). Those languages were created and designed by people, and we can make new ones! That's what this is about.
Programming languages are really cool, because it's not just about communicating our ideas to the computer so it can execute it, but it also provides a great opportunity for the computer to tell us when we're goofing up, or making mistakes and design errors.
No worries! It's an exciting and fascinating space to work in, and I wish people knew more about it! Even professional programmers can sometimes take their programming languages for granted, and not consider them as objects of study and design.
These kinds of mistakes can lead to your passwords getting hacked, or causing bugs and crashes etc. Some of the really cool work that is being done in PL is about leveraging the computer to help us avoid those common, silly, human mistakes.