Friday the Senate will vote on final cloture (60 vote threshold) for a House bill/Continuing Resolution that funds everything in the government except for Obamacare. Earlier this week, Senators voted 100-0 to proceed and bring the bill up for a vote today. Majority Leader Harry Reid (if he gets cloture first) is only requiring 51 votes to amend the House bill in order to put Obamacare funding back in, which would require no votes from Republicans. Usually controversial and important amendments require 60 votes, but because Reid doesn't have enough, he's going with the 51 votes option.
Republican Senator Ted Cruz has been arguing all week that if Reid gets enough cloture votes today to end debate on the House version of the bill, he'll be in an unabated driver's seat to immediately amend the bill, take out the language defunding Obamacare and replace it with language funding Obamacare, all with zero Republican votes.
Cruz has also been pleading with Republicans to stand together and vote against cloture in order to continue debate, which would pressure Reid into requiring 60 votes instead of 51 to add Obamacare funding.
The bottom line: If Reid gets cloture on the House bill, debate will end and then he'll have the ability to put funding for Obamacare back into the bill with just 51 votes. Because of this, the only real vote that matters is a vote on cloture. Republicans who vote for cloture on the bill today are helping to end debate on the issue are helping Reid with his plans to fund Obamacare.
When the bill goes back to the House with Obamacare funding (which it will, some Republicans will be voting with Reid for cloture), Representatives will take it up tomorrow (Saturday) and have said they will not accept anything with Obamacare funding.
Congress has until Monday afternoon to figure something out before a government shutdown.
Voting is expected to start at 12:30 ET. You can watch live here.
UPDATE: Dan Holler from Heritage explains.