The SCOTUblog plain English summary;
Today the Court issued its decision in Shelby County v. Holder, the challenge to the constitutionality of the preclearance provisions of the Voting Rights Act. That portion of the Act was designed to prevent discrimination in voting by requiring all state and local governments with a history of voting discrimination to get approval from the federal government before making any changes to their voting laws or procedures, no matter how small. In an opinion by Chief Justice John Roberts that was joined by Justices Scalia, Kennedy, Thomas, and Alito, the Court did not invalidate the principle that preclearance can be required. But much more importantly, it held that Section 4 of the Voting Rights Act, which sets out the formula that is used to determine which state and local governments must comply with Section 5’s preapproval requirement, is unconstitutional and can no longer be used. Thus, although Section 5 survives, it will have no actual effect unless and until Congress can enact a new statute to determine who should be covered by it.
The formula was originally passed in 1966 as a counter to Jim Crow laws and so targets only those areas who had such laws in place. This was originally only supposed to be law for five years, but has been renewed by congress repeatedly without any changes. It's this formula that was found unconstitutional as opposed to the rest of the law. Section 5 (pre-clearance for such voting restrictions) remains on the books, but without the formula as to where it should be applied it simply isn't applied anywhere.
Theoretically congress could simply create a new formula or make section 5 universal. I have a feeling that the latter might be found unconstitutional as well, given some of the language I've glanced at so far, and any new formula would be political toxic waste. It would basically be a way of telling parts of the country they are too racist to be trusted.
So it effectively ends such pre-clearance altogether even as it says that such clearance might well be required and is theoretically constitutional. Nice trick there.
As for why such pre-clearance is important, imagine if a flat out illegal voting restriction is passed. To get it overturned you need to fight a court case. Unless you get very lucky that court case might take longer than the next set of elections. So such a law can easily have an effect on the outcome of an election even if it is then rapidly struck down. Given that states control voting regulations for federal elections held within their borders, such a law could easily effect who gets into federal office. And given some of the tricks that certain states have been trying over the last few years it's not a reach to imagine such a thing happening.
EDIT: First effect analysis I've seen;
What this means in Texas.Basically Texas had two new laws - redrawing voter districts and voter ID - which had been pre-challenged under section 4/5 and were under appeal to the Supreme Court. At least the voter ID law can now be legally enforced despite the racial problems suggested by the challenges. It could still be retroactively challenged but the odds are very good that they would be in place during at least one election. The situation with the district maps is more complicated but at least one major hurdle has been removed.
EDIT 2: Texas is now looking to implement the voter ID law passed in 2011. This law had been blocked under pre-challenges that are now moot. Essentially the laws were already found to be illegal discrimination, but that doesn't matter until someone who is blocked from voting can bring a new case.