The growth rate is the change in Gross Domestic Product at Constant Prices, that is it removes the effect of price inflation (or deflation) and indicates how much more a country is really producing compared with an earlier period.
It is normally shown in percentage terms. So if we say that the economy grew by 5% then for every hundred widgets produced the previous year, 105 are produced this year.
The growth rate is an important indicator of how well an economy is doing. Because GDP is calculated the same way across countries, their growth rates can be compared.