Region Level Cap is when you lock a Pokemon's level to a selected region, even if that Pokemon is in a different region with a higher level cap, its level is still aped to the selected region.
Question & Answer
Q: How do I unlock it or do I have to pay for it?
A: It is a free feature and you unlock it after the 4th badge on the first region.
Q: How helpful is this feature?
A: For example you want to EV train your Pokemon so you go to a different region and start EV training your Pokemon, but you end up with an over-levelled Pokemon and you can't use it.
Q: How to use the Region Level Cap?
A: On the summary screen next to the Pokemon's level there is a lock icon, clicking (or touch) on it you can select the region.