Flex is a term used for how flexible the stick is, the higher the flex number, the stiffer the stick. Choosing the right flex is important because the right amount of flex can improve the accuracy and the power of your shot a great deal. Having too much can have a negative effect!
As a thumb rule you should choose a flex equivalent to your weight, depending how tall and strong you are, and add +5 to +10 flex. Meaning if you weigh 80 kilos you should have a flex 85 to 90. But it is only a thumb rule and a guide line, so try different styles and flex before finding the perfect flex that match your playing style and level.
Important note! When you cut down your stick to match your size, the stick will shorten and the flex will increase. So if you buy a flex 90 but cut it to fit, you could end up with a stick that is flex 100!