Sorry you received unhelpful sass from other commenters. It stands for "Objective and Key Result" - it's a common "business-speak" initialism that basically translates to "a measurable, data-driven goal".
I'd say it's more than that. OKRs are a concrete framework that came out of Intel and was popularized by Google. But yes, the focus is on setting objectives backed by measurable results.
Can be a document, an app, whatever. One flavor is objectives are set at the top, their KRs are translated into objectives for the next management layer with their own KRs, and so on. It's an iterative process with some combination of top-down and bottoms-up activities. And afterwards, the results are reviewed. There's more to it, but that's the gist.
An objective is a qualitative goal. The key results are generally quantitative numbers that can help you measure if you are achieving that goal.
An example: an objective might be "improve application performance". The key results for that objective might be "reduce average page load time from 3s to 1.5s" and "reduce API response times by 60%".
If you were to tell your CEO you're reducing page load time, they will rightfully ask you why and what problem you're solving. The objective is the simple sentence that makes the "why" clear. It might itself be subordinate to a higher level objective. The idea is that all work is clearly aligned to specific goals in a very transparent manner.
It's actually a very solid framework, but as the many comments indicate, poor implementations abound (also see Agile, DevOps, etc).
The point is that you set a goal and made progress towards it. Ideally you choose goals for yourself and it's expected that you won't complete them all.
My experience of them was that upper management set "Objectives" like "1% YoY Online Sales Growth" for your group or team or whatever and then each team in that group (again, or whatever) would come up with their own "Key Results" that they thought, if the result was achieved, would translate to that objective.
So for example, if you worked in online retail like I did, maybe you'd get an objective like that and then hypothesize a few things along the lines of "if we increase product image interactions by X% then sales should go up by Y%." as key results that you then report back out to management to show progress towards your objective.
update: dead serious. I searched. It says "objectives and key results." But what does one look like? Is it a document?