Revenue is the money coming into the business from customers, before any expenses are taken out. It is sometimes called the top line, because it sits at the top of the profit and loss with everything else subtracted below it.
Revenue is not cash. Work invoiced but not yet paid counts as revenue while the cash is still sitting in money owed to you.
Growth rate
Revenue growth rate compares a period against the one before it. For seasonal businesses the useful comparison is the same period last year rather than last month, since a slow January says nothing about a business that does most of its work in summer.