Redundancies are words that can be eliminated from your text because they repeat what has already been expressed or conveyed elsewhere in the sentence.
He reversed the car back into the driveway.
can be simplified to:
He reversed the car into the driveway.
He threw his keys onto the table.
He threw his keys on the table.
The roof outside was covered in snow.
The roof was covered in snow.
Eliminating redundancies helps make your prose leaner and cleaner, which improves readability.
The exception to the rule
While most redundancies should be rephrased, you don’t have to eliminate every last one. An occasional redundancy is okay, especially if it works in context—for example, you may have a character who uses them in dialogue.
The Bottom Line
Strong writing is crisp and distinct. Redundancies are amateurish and weigh down your sentences.