Shaft flex is the firmness of the shaft and/or resistance to bending- shaft flexes are L (ladies). A (Senior), R (Regular) , S (Stiff), and X (Extra Stiff). As you go from L to X the shaft feels firmer as they get harder to flex/bend. Also shafts generally get heavier as you move from L to X. There are no standards between manufacturers, thus, as an example a manufacturer may designate a shaft stiff that another manufacturer may designate Regular. The same situation may also occur within a given manufacturer having several available models- crazy!!

Once you get beyond the beginner level it will be important to be fitted for the proper shaft for your swing to get the best performance. I hope this helps, however, I expect you will receive many useful comments.

Besides the stiffening from L to X, there are other key characteristics where the shaft flexes the most and the overall weight. More reason why the flex letter is of no value to compare between 2 shafts and why a fitting is so valuable.

Precise answer is impossible as there is no industry standard. But senior flex is the softest, and stiff is obviously harder than regular. What it means. It means that if you have slow swing you should play regular or senior. If you have fast swing speed, you should play stiff or extra stiff. That is a general rule. It gets more complicated as 60g stiff graphite shaft plays softer than 100g regular iron shaft. Then, aggressive swingers need more stiff shafts, than smooth swinger despite the same swing speed... and we did not touch yet the torque, kick point, swing weight or transition etc...
Go to fitter for optimal shaft. Or at least get your driver or 7iron swing speed measured somewhere and a good shop assistant looking at you and knowing your golf skills should get you proxy good shafts.