Commands:
/statistic (alias /stats).
Usage: /statistic <player> <stat> [argumentValue] [value]
Argument value is if the statistic is about a certain block, item, or entity, for example MINE_BLOCK. If you are setting a statistic with no argument, usage is /statistic <player> <stat> [value]
If you want to read a statistic but not change it, omit the value parameter.
If you want to see all of a player's stats, you can use /statistic <player> ALL.
To change a stat by adding or subtracting, put ++ or -- in front of the value, respectively. Like:
/stats Player DEATHS --2
would subtract two from Player's death stat.

The permission for using this command is statisticeditor.statistic, the permission for changing statistics with this command is statisticeditor.editstatistic.

/runifstat
Usage: /runifstat <player> <stat> [argumentValue] <operator> <number> <command>
As explained in /statistic, argument value is only present if the specified statistic requires it.
Example: to give the player datatags_857 $100 (using essentials) if they've killed less than five bees, run:
/runifstat datatags_857 KILL_ENTITY BEE < 5 eco give %player% 100

This plugin supports PlaceholderAPI, the placeholders work like this:
Argumentless stats: %stat_DROPPER_INSPECTED%
Stats with arguments: %stat_MINE_BLOCK-BEDROCK%

Want to know what all the stats do? Check out the Minecraft Wiki page on Statistics! Unfortunately, the Spigot statistic names don't quite match the Vanilla names in all cases, so I've made a couple mapping pages on the StatisticEditor wiki:
Spigot -> Vanilla statistic mapping
Vanilla -> Spigot statistic mapping
There's also a configuration option you can enable so the plugin uses the Vanilla mappings instead of the Spigot ones, but it's sort of experimental so far.