Specific impulse

No problem man, if the other videos are not translated, there's (hopefully) wiki pages in your language

@PointBreak I found the translated video, thanks for the reply

@PointBreak sorry, but my level of English is too low to understand this video, but I'm too lazy to translate (I'm very lazy)

It's efficiency. Diferent fuels have different Isp. Same with different engine types. You can increase it by using a hydrolox instead of kerolox for instance. Lowering the nozzle throat size can also improve it, but it decreases thrust. Nozzle length is also important and usually the vacuum varians are longer than sea level variants.
You should check out Scott Manley. In this one he talks about Isp.

i believe it's something to do with the fuel efficiency, i'm not entirely sure though, but since it's measured in seconds i'm guessing it's how much time it takes for the engine to consume a given amount of fuel (not specifically the amount it is connected to, since a non-XML rocket engine still usually gets around 150-350 isp regardless if a normal or semi-infinite fuel tank is used)

isp defines how well a rocket preforms