Karma is a temporary stat earned during story-heavy events. It's best explained as your reputation during that specific event - actions that can be seen as beneficial, moral, and/or lawful to a faction would be Positive Karma, while actions that would be hindering, immoral, and/or chaotic to a faction would be Negative Karma.
Karma can be obtained via certain activities (listed in the next tab - some may be undiscovered) while a Karma Event is active. Some activities can only be done once per Rixixi and can be therefore claimed retroactively if you've already done it - this will also be listed in the next tab.
Both Positive and Negative karma for a Rixixi max out at 100, and gaining Positive karma doesn't decrease your Negative karma (and vice-versa.) For instance, you could consider having 50 Positive and 50 Negative karma "neutral" karma (though a somewhat unpredictable neutral).
A Rixixi's Karma resets after every Karma Event - you can claim retroactive karma again for each one, and max out your karma each one. This is because it's used in the Karma Shop for prizes! You can redeem Karma Milestones throughout the event as you reach certain amounts without using up any Karma at all, but at the end of the event, new items will be revealed in the Karma Shop that you can use your User Currency on - typically Karma Shop-exclusive or just rare ones.
You can, however, see a record of your Rixixi's previous Karma from previous events - while your Ixi's karma effectively 'resets' back to zero at the end of a Karma event, their records will have frozen at their final point.
A user's Karma is earned when any of their Rixixi earns Karma and does not max out at 100. It also does not reset post-event, and can be turned in at any time for prizes - event shops don't close. You can choose to turn in all of your pooled Karma at once, save some for the next event, or turn it in whenever you like, it's up to you.