This expression has the same meaning as the English "They promise us the earth/moon", meaning that someone is making extravagant promises they either will not or cannot fulfill (promising miracles).
Basically, they're just telling people whatever they want to hear, as a means to an end. It's typically used about politicians and the like, who tend to make promises during the election, but fail to follow through once they're elected.
It's the same as "Promettre monts et merveilles" in French.
(I'll edit the translation to make this all a bit clearer.)
In Polish it can be translated as [infinitive] "obiecywać gruszki na wierzbie" - literally: to promise pears on a willow", and not completely literally: "to promise pie in the sky" - promise something completely unreal/impossible.
I'm not 1000% sure about "pie in the sky", if somebody from UK can confirm that - thanks. :)