The details of the compromise aren’t totally clear, but will include reducing the 60-day review period of the Iran deal to 30 days, though the period would be extended under certain circumstances, including if the deal is submitted during the summer recess, as it well may be.
The White House had long threatened a presidential veto on the bill, and the backing down seems to reflect the reality that the bill is going to have enough support to override such a veto. White House officials did express reservations about linking the nuclear deal with terrorism, however.
The broad purpose of the bill is to give Congress the ability to veto an international deal with Iran. Israel has been pushing hard for them to have such a power, since they would almost certainly block anything Israel objects to, which is basically any deal at all.
Apparently resigned to the likelihood of a veto override at any rate, Senate Democrats had been working with the White House to try to weaken the language of the bill in hopes of making it somewhat more palatable. Whatever the final language, it is a serious blow to the negotiations, and will make it harder for the US negotiators to credibly pledge to keep up their end of any deal.