こんにちは (Konnichiwa) Nicholas1024 and ssorgatem,
As ssorgatem said, せんせい (sensei) can also be used as an honorific title. As a noun, it means “teacher,” but when you add it after someone's name, it politely refers to them using the title of “teacher.” In English, this usually gets translated as “Mr.” “Mrs." etc, but literally, たなか せんせい が せんせい です (Tanaka sensei ga sensei desu) would mean “Teacher Tanaka is (the one that is) the teacher.”
You could also say たなか さん が せんせい です (Tanaka-san ga sensei desu), but by addressing Tanaka using the title of his profession (i.e. a teacher) instead of さん (san), we can show a lot more respect.
Think of how you might talk about a teacher in English. You wouldn't say “Smith is my teacher," but you would say “Mr. Smith is my teacher," right? It sounds a lot more respectful.
Note that せんせい (sensei) can also be used to refer to other professionals, like doctors. So you could also use たなか せんせい (Tanaka sensei) to say “Dr. Tanaka." Don't worry, you'll be able to tell which is which from the context!
I hope this helped! Please let me know if you have any other questions.
べんきょう を がんばって ください！ (Benkyō o ganbatte kudasai!)