I discovered the transactional analysis through the book "Games People Play" by Eric Berne. ( really good book, I can really recommend anyone to read it )
Someone gave me the book saying they had to read it for university, but soon I discovered how genius this book is.
The transactional analysis is basically the scientific breakdown of all human interactions.
Eric Berne was a psychiatrist who discovered that basically all human interaction can be broken down into games as he called it and the goal of those games was always to get a "transaction" which he defined as a unit for measuring human social interaction.
The reason why people need those transactions is, that humans not only need physical food to survive, but also social interactions, that are measured in the "transactions" for simplicity sake.
Most people that read the book probably have to do it for their education in business, marketing, psychiatry etc since it helps a lot in understanding how you have to interact with people to achieve a certain result and so they may not get the deeper meaning and the genius behind it, they just see it in a limited way, but if you look deeper it is in reality the holy grail of understanding human behavior, that is also why I write about it, since I think most people don't get the full conclusion of this discovery, which is much more interesting then what people normally use it for.
This should have been a very short description of what transactions are, but even though it is very short, I probably spoiled it already since I gave the final conclusion away, but maybe this is needed as well, since otherwise most people likely will not bother to look into it in the first place. The book itself is very short as well, most of it just describes the specific games and the deeper meaning is just covered by a few paragraphs, this is why I want to extend that part myself and continue the research.