In standard media accounts, the resolution is being described as an attempt to move the "red line"--the line that, if crossed by Iran, could trigger a US military strike. The Obama administration has said that what's unacceptable is for Iran to develop a nuclear weapon. This resolution speaks instead of a "nuclear weapons capability." In other words, Iran shouldn't be allowed to get to a point where, should it decide to produce a nuclear weapon, it would have the wherewithal to do so."
He goes on to point out that "capability" is rather a slippery word. Iran probably is capable now to produce a nuclear bomb or two within two years if it wanted to. One interpretation of Lieberman, Graham, and Casey's resolution is that it would call for war immediately on that basis. The three are also trying to legislate against using a containment strategy in the event of a nuclear armed Iran (you know, that thing that prevented a nuclear Holocaust during the cold war). They want the Senate to resolve that it:
Strongly supports United States policy to prevent the Iranian Government from acquiring nuclear weapons capability; rejects any United States policy that would rely on efforts to contain a nuclear weapons-capable Iran; and urges the President to reaffirm the unacceptability of an Iran with nuclear-weapons capability and oppose any policy that would rely on containment as an option in response to the Iranian nuclear threat.
That amounts to demanding immediate war with a nuclear-armed foe whose conventional forces would be helpless against the US. No negotiation, no wiggle room, no jaw jaw. Just war war. Lieberman made their intent clear in January:
"When it comes to addressing the threat of a nuclear-armed Iran, all options must be on the table -- except for one, and that is containment. We are confident that an overwhelming bipartisan super-majority of our colleagues will join us in passing this resolution, which will send a clear message to Iran's rulers that we are absolutely determined to stop them from getting nuclear weapons. Containment is failure, and failure cannot be an option."
That's the sort of thing that might lead to a nuclear detonation.