Buying ski equipment for the first time is exciting, but can be confusing because there are so many options. Also because what seem like standards are not really standard across brands. Just as slope ratings (green, blue, black) are only guidelines relative to a specific ski area, boot flex is a guideline within a brand. (A black run in a small southeast or midwest ski area can be more like a blue run somewhere in the Rockies or northeast.) A boot flex of 70 is softer than 100 for a given brand, but an intermediate buying boots for the first time should not use flex as the only criterion for picking a boot.
Check out this discussion of boot flex:
Below is a link to a good short article about the basics of buying ski equipment for a beginner or anyone ready to move away from renting boots, skis, and poles for fun on the snow. At the top is important advice:
"The prevailing advice from ski instructors and shop sales persons is that beginners should rent before they buy, and that they should first invest in ski boots and purchase their first pair of skis later in the process."