That there is no بدا ie our destiny is set and it can't be changed.
In Arabic, the word hand implies power/ability.
و في التّوحيد عن الصّادق عليه السلام: في هذه الآية لم يعنوا انّه هكذا و لكنّهم قالوا قد فرغ من الأمر فلا يزيد و لا ينقص قال اللَّه جلّ جلاله
تكذيباً لقولهم غُلَّتْ أَيْدِيهِمْ وَ لُعِنُوا بِما قالُوا بَلْ
يَداهُ مَبْسُوطَتانِ يُنْفِقُ كَيْفَ يَشاءُ ا لم تسمع اللَّه تعالى
يقول يَمْحُوا اللَّهُ ما يَشاءُ وَ يُثْبِتُ وَ عِنْدَهُ أُمُّ
From: Tawhid al-Saduq (The most important Shia book on Tawhid, written by Shaikh Saduq):
About this verse...it doesn't mean that the Jews said that his hands are physically tied, rather what they said was that Allah is free from giving orders and settings things and there is no addendum to his work nor anything is to be removed.
To Refuse them Allah had said: "(verilly) they are the ones who their hands are tied (can't change things) and because of this lie they became helpless. Indeed Allah hands are free to give away (انفاق) however he pleases. Have you not heard of Allah in Quran saying: Allah eliminates what He wills or confirms, and with Him is the Mother of the Book.
So basically (with a mocking tone) they were saying Gods can't change things. Why?
The Muslims were in a terrible financial shape. So the Jews were mocking Allah and saying his hands are tied and he can't help the Muslims. That year itself was also a year of drought, so the it was painful to the jews as well. Additionally when the heard the verse of Who is it that would loan Allah a goodly loan so He may multiply it for him many times over (Baqara verse 245) They started mocking the Muslims and saying what kind of a God is he that he needs to borrow from his servants?
Hence came the answer and the curse. That Allah hands are extended. Later things flourished for Muslims and it got worse for the Jews.
I found the story from Tafsir Shaikh Tusi