In the introduction, the Alter Rebbe writes a text to the people who will study Tania.
The Alter Rebbe poses a problem: there are two types of books.
1) When an author writes a book, he writes ideas that he has tested and found useful—his own practical ideas. Just as when a doctor writes a medical book, he writes about the things he has seen and tested based on his experience. But this is only useful to another doctor of the same level of knowledge as the one who wrote the book! In the same way, if the Alter Rebbe wrote a book with ideas that served him well in his service to Hashem, Who says it will work for us too? We're nowhere near the spiritual level of the Alter Rebbe!
2) There are books that are for everyone, like the Torah and their commentaries, as well as the explanations written by great rabbis at different times. But the problem with these books is that it is very difficult to find in them the solutions to the problems in the service of Hashem of each one. They are very general books and it is difficult to find answers in them to specific questions that the reader may have.
The Tanya: What kind of book is it? If it's the first type, it's useless because we're nowhere near the spiritual level of the Alter Rebbe. If it's the second type, it's useless too! Who says we'll find answers to our questions there? So, why did he write the Tanya?
The Alter Rebbe explains that one of the reasons he wrote the Tanya is so that his followers (Hasidimso they don't have to ask him about every matter in his service Hashem (avodat Hashem) in person. Instead, they could find the answers to their questions in Tania.
But that would only work if the Alter Rebbe knew everyone who would read the Tanya: and that's precisely the point. The Alter Rebbe says that he knows all his followers and that the Tanya was written for each one of them—that is, the answers to the questions in the service to Hashem Each one of them is in Tania, including the answers to our questions!
The Alter Rebbe clarifies that those who cannot understand the Tanya on their own should ask more experienced and knowledgeable followers. He also warns his experienced followers that if they do not share what they know, they will receive a punishment of Hashem But on the other hand, if they share what they know, Hashem It will make both the one asking and the one answering smarter people.
Initially, the Tanya was written in small pamphlets, but people copied them by hand and made many mistakes. So, the Alter Rebbe decided to have it printed, compiling all the material into a book, to eliminate the possibility of errors.