Softball rec league coaches probably do not have to worry about the DP/FLEX. Most rec leagues allow open substitutions so that everyone gets a chance to play. However, a rec league All-Star or Travel ball coach should learn how the DP/FLEX works, and how it is so much more powerful than just a Designated Hitter.
The DP is somewhat similar to the Designated Hitter in American League baseball. She bats for someone in the field (usually, but not always, the pitcher), but does not play in the field. The "Flex" is the player for which the DP bats, so the Flex plays in the field but does not bat. If your DP always bats for your pitcher and never takes the field, then she is doing exactly what a baseball DH does.
Where it gets interesting is when the DP/FLEX concept is combined with the substitution rules for softball, which are completely different than baseball. A Softball coach that understands how to combine the two to best advantage has a HUGE tactical advantage over a coach who does not. Baseball and Softball substitutions are similar in that any eligible player on the bench can replace a player in the lineup, and any player in the lineup can swap positions in the field with any other player in the lineup.
Where Softball substitution differs is that a starting player can re-enter the game once if she is substituted for (the substitute, once she leaves the lineup, is out of the game). There are also courtesy runners who can run for the pitcher and the catcher, but are not considered substitutes or pinch runners. A player can start the game as a courtesy runner and later substitute for another player in the lineup (once she does so, though, she cannot be a courtesy runner anymore and is out of the game if she leaves the lineup).
The Flex can bat for the DP, but that is considered a substitute for the DP (and drops the number of players in the lineup from 10 to 9). The DP can then re-enter the lineup (because she was a starter), but she has to re-enter in the same slot in the lineup that she started. Similarly, the DP can substitute for the Flex, in which case the DP plays in the Flex's position in the field, and again our lineup drops down from 10 players to 9. Basically, the DP and Flex are joined at the hip -- they can only substitute for each other in the lineup. I like pairing my best power-hitter-but-weak-fielder with my best-bunter-but-weak-hitter defensive player, and then having my Flex bat if I *need* a bunt to be laid down when it is the DP's turn to bat.
The big difference between the Baseball’s DH and Softball’s DP is that the DP can swap defensive positions with any other player in the lineup just like any other player in the lineup can. Say the DP happens to be a team’s backup catcher, and the starting catcher is gassed because she has caught three games already today. The DP can spell her for an inning or two catching, while the starting catcher (now in position F10) stays in the lineup to hit. This is the real beauty of the DP/Flex -- the DP acts not only as a "designated hitter", but as a reserve defensive player available whose use does not burn a substitution.
In my opinion, a baseball manager has to manage the limited resources on his bench more carefully than a softball manager does, but a softball manager has to be more aware of the strengths and weaknesses of her bench (and DP and Flex players) and how they pertain to the current game situation. A smart softball manager can move players in and out of situations to maximize success much more than a baseball manager can.