The rate of a reaction tells you how fast it happens. For an enzyme-controlled reaction, rate = amount of product formed ÷ time taken. You can also use the amount of substrate used up in the same way.
Worked example: an enzyme makes 24 cm3 of oxygen in 60 seconds. The rate is 24 ÷ 60 = 0.4 cm3 per second. In another experiment 30 cm3 of gas is made in 5 minutes, so the rate is 30 ÷ 5 = 6 cm3 per minute. Collecting 45 cm3 in 90 seconds gives 45 ÷ 90 = 0.5 cm3 per second. Always give the unit, and make sure the time unit matches the one in the question. If the time taken for the same amount of product halves, the rate doubles.
On a graph of product against time, the line is steepest at the start, because that is when the rate is fastest. The line then flattens as the substrate is used up. If the enzyme is heated above its optimum temperature it is denatured and the rate falls.