What is kn/s?

kilo nodes per second, I think.

From the german help file of Shredder 11:

Viele Schachprogramme stellen im Analysefenster die Information ”Knoten pro Sekunde” dar. Diese Masseinheit wird allgemein als Information interpretiert, wieviele Stellungen ein Schachprogramm pro Sekunde bewertet.

Schnelle Programme bewerten auf aktueller Tophardware über 1 Millionen Stellungen pro Sekunde.

My quick attempt at translation:

Many chess programs display the information "nodes per second" in the analysis window.
This meassurement is generally interpreted as information how many positions a chess program evaluates per second.
Fast programs on current top hardware evaluate over 1 million positions per second.

1kn/s = 1000 moves Per second (Chess calculation ability)

where exactly did you get this from?   1node = 1000 moves ?

Well, kilo means 1000 (kilo=1000) so I guess that's the logic. 

Keep in mind the number being displayed by the engine is completely subjective to that engine's design. In order words, 12,000Kn/s on Stockfish is not equal to 12,000Kn/s on a different engine. So the number is pretty much useless information except when comparing the same engine on different hardware.

So how do you determine the strength of various engines?

lol

knots per second