First of all, to answer this question you will need to know what a clause is.
The basic definition of a clause is a group of words that contains a subject and a verb.
For example: Although I wasn’t feeling well, I went to school.
Another example: They went inside.
In the first example, “although I wasn’t feeling well” is a dependent clause. (It doesn’t make sense as it’s own sentence. However, “I went to school” is an independent clause. (It can be it’s own sentence)
In the second example, we have one independent clause. “They went inside”
A simple sentence consists of just one independent clause. Example:
1) They went inside
2) I went to school
A complex sentence consists of a sentence containing one independent clause and one (or more) dependent clauses. Example:
1) Although I wasn’t feeling well, I went to school
2) While I am a passionate basketball fan, I prefer football