"Das geht ab!"
This is slang/dialect and has nothing to do here in these lessons! Noticed that, the translation is wrong/misleading (full stop)
Slang Explanation: Some may say: "Das geht ab, wie eine Rakete", or, "Das get ab wie Schmitt's Katze", or "Das get ab wie ein Blitz" To say that something takes off fast, or quickly; or maybe it is just fun or bonzer. (Australian slang)
"Das geht ab" on it's own is rarely used in German anyway, mostly with the above comparisons, and yes - as said above, it has mostly to do with speed.
I am native German speaker, I wouldn't teach this is the early lessons at all, this is mean of Duo.
And the other meaning of this one is: (it can mean this can be undone, peeled off, washed or sweeped off) = "Das geht ab"
For example: Someone puts a sticker on your car and you question him what this is all about, and he replies: "It is easy to get it off again!"
Which will sound in German: "Das geht ab". Or better I should say:
"Das geht wieder ab!"
This works only one way and has nothing to do with speed, it just means it can be removed again.
Gehen stands here for: be able to be removed
ab stands for apart, away, off
Hope that helped and did not confuse more. :-)
The meanings given in the drop down boxes seem to bear little relationship to the 'correct' answers given. The drop down boxes say 'ab' means 'away' or 'from'.......which suggests that "Das geht ab!" means "That goes away" rather than "That is fast" which I'm guessing would be "Das geht schnell!"??