A market researcher at a major clothing company that has traditionally relied on catalog mail-order sales decides

Question:

A market researcher at a major clothing company that has traditionally relied on catalog mail-order sales decides to investigate whether the amount of monthly online sales has changed. She compares the mean monthly online sales of the past several months with a historical figure for mean monthly sales for online purchases. She gets a P-value of 0.01. Explain in this context what the 1% means.