Theory of computation is the study of abstract machines and the algorithms that they implement. It is a branch of theoretical computer science and mathematical logic, and is also known as computational theory or TCS. Theoretical computer science is a broad field which includes the study of algorithms, data structures, complexity theory, cryptography, parallel computing, distributed computing, artificial intelligence, machine learning, natural language processing and more. As you can see, there is a lot to learn in this field!
That being said, theory of computation is not necessarily difficult to learn. It depends on your background and what you are interested in. If you are already familiar with mathematics and formal reasoning, then learning the basics of TCS will likely be quite easy for you. However, if you are not used to thinking about abstract concepts like algorithms or complexity classes, then it may take some time for you to get comfortable with the material. In either case, it is important to start by getting a good understanding of the basic concepts before moving on to more advanced topics.
There are many resources available for those interested in learning more about theory of computation. There are textbooks which cover the topic in depth, online courses which can be taken at your own pace, and lecture series which provide an introduction to the subject matter. Whichever route you choose, make sure that you put in the effort to really understand the material – it will pay off in the end!