It depends onwhat you really mean.Here the underlined clause is a/an -

It depends on what you really mean. Here the underlined clause is a/an - Noun clause.
A noun clause is a group of words that band together and act like a noun. Nouns clauses are used when a single word isn't enough. They're always dependent clauses.

বাক্যের চারটি স্থানে Noun clause বসতে পারে -
1. subject হিসাবে
2. transitive verb এর object হিসেবে
3. Linking verb এরপরে complement হিসাবে
4. Preposition এরপরে
প্রদত্ত বাক্যে " what you really mean" clause টি preposition এর object হিসাবে বসেছে, তাই এটি Noun clause.