A simple sentence is one which contains only one subject and one predicate.
Subject: Person or thing about which something is said.
Predicate : What is said about the subject.
In other words a simple sentence contains only one main clause. It does not have subordinate clauses.
Chandigarh is a planned city.
My father is a senior manager in the organization.
Microbiology is an interesting subject for me.
Note that there is only one finite verb in each of the above sentences. That means there is only one clause.