‘Flex’ is a ski boot’s ability to bend as you move your knee and ankle joints. If the flex of your boot is high, your leg will feel supported and it’ll be easier to control the direction you’re going in. If the flex of your boot is weak, you’ll have access to a wider range of movement but need the strength to hone it. You need to find the right balance for your body and ability.
When you’re shopping for your new ski boots, you’ll find the flex is determined by a number that can be found on each product page.