The Shimano Baitrunner D Series Reels is the standard in baitrunner spinning reels. The baitrunner feature allows you to pin a bait on, cast the bait, then flip the lever on the back of the reel. This will put the spool into free-spool allowing the bait to swim while still having the bail closed. The problem of fishing a spinning reel without this feature is if you get bit by a larger fish, you can't close the bait because the line is coming off the reel too fast. If you can't close the bail, you can't catch the fish.
The dial on the back of the reel allows you to increase or decrease the freespool drag. Meaning while the lever is pressed down, you can slow a big bait if it's swimming too fast or you can free up the spool with a smaller bait. Once you turn the handle, the main drag will engage and you can fight the fish.