That STR-F determines how much the effect of that specific skill will be affected by the user's Strength stat.
There isn't a way in the default setup of RMXP too temporarily increase a stat. For all my buffs and debuffs I just make a status effect for each one. Make a new status effect and call it Buff or whatever you want for the STR-UP buff. The only thing you have to change is where it says Strength, put the percentage you want it to put the character's Strength stat to. For example, if you want the character's Strength to double, put 200 in that box. You should also change the recovery conditions down at the bottom to whatever you want. If you want it to end after 3 turns, set it to 100% chance of recovering after 3 turns.
Now, make a new skill that you want to increase Strength. Make all the stats 0, except for SP (put however much MP you want it to use) and Accuracy (put that at 100). Now over on the right find your Buff status effect, and put a + in the box next to it. Now name it whatever, put whatever icon you want, put who you want it to affect etc.
Edit: There ARE scripts that will let you make skills that do all kinds of cool things...but I don't use them cuz they aren't compatible with my other scripts. Even though I would LOVE to be able to have more dynamic skills...I just live with it, no big deal. But you might want to try that, just search for "status" or something in the Scripts section.
