Economy Plus, Main Cabin Extra, Economy Comfort, each airline calls it something different, but for all intents and purposes, it's Economy/Coach, with a little extra legroom. These are usually the first few rows of the coach cabin. The quickest way to secure these seats is to book your economy ticket, and while selecting a seat, an additional fee is added if you want one of these rows.
I try not to pay extra for these seats and will pick a center seat if it means I don't have to pay extra. But a few times I have been able to be placed in these extra legroom rows without paying extra for them. A few factors have to perfectly fall into place, but if you keep your eyes open between booking and wheels up, you, too, can find a little extra space for no cost.
When booking your ticket, the only available seats may be in these rows that cost extra. If that's the case, do not select a seat at booking! Proceed with an "unassigned" seat and purchase your ticket. When you check in for your flight, if doing it online, sometimes the system will ask if you want to select a seat. Look at the seating chart again. If the only available seats are in this "extra" section and the system is still noting that they'll charge for them, then forego the seat selection. Just check in and the airline will assign you a seat. Chances are it'll assign you to the only open seats left: those in the extra legroom section... and won't charge you.
However, if during check-in you notice that there are other available seats besides the extra legroom ones, seats that have no additional charge, then you'll likely be placed in the regular seat if you let the airline system select for you. Just keep an eye on the inventory and you may just luck out.