You use the phrase "a sharp decline" when reporting numbers like sales numbers, statistics, etc. It means that the numbers decreased suddenly.

For example, if a company has been making ten million dollars a month, then suddenly makes only seven hundred thousand dollars one month, that is a "sharp decline" in the company's earnings.

Use this phrase in a sentence like this:

There's been a sharp decline in the rate of violent crime since she took office as mayor.

We've seen a sharp decline in DVD sales this year.