what does the symbol "≡" means? and how to read this equation a ≡ b?
It really depends on the context.
If you are working modulo some number n, then means that a and b are congruent modulo n, which in turn means that n divides the difference a-b. For instance, , since 2 divides the difference 7-3.
If a and b are functions from the same domain, it could also mean that the two functions are identical, i.e. for all values of x in the domain, we have that a(x)=b(x). For example, the two functions and are identical on the set of real numbers.
It might not be one of these two options, though. Some context will help in answering the question.