Why is there an election right now?
Unlike in the U.S., the British prime minister has the power to call a "snap" election, and May did so in an attempt to increase the number of Conservatives in Parliament. With more members of her party in the government, it would be easier to pass the legislation needed in order for the U.K. to officially leave the EU.
Who's in the race?
May is still leading the Conservative Party, (also known as the Tories), continuing her commitment to deliver what Brexit voters chose last year. She wants to significantly reduce immigration and asylum claims, while still allowing some refugees to enter the country.
The other major political party is the Labour Party, led by Jeremy Corbyn. He's also committed to carrying out Brexit, but wants the U.K. to stay in the single market and the customs union, which allow EU countries to freely trade goods and services, and limits tariffs on imported goods. (However, German Chancellor Angela Merkel has said Britain is "under illusions" if lawmakers thinks they'll keep EU benefits after exiting.) In terms of immigration, the party and Corbyn don't agree with May's plan to drastically decrease immigration.
Why does this matter for the U.S.?
The U.K. is going to leave the EU, but which party's leading the country will determine how it goes about exiting. Labour has proposed keeping certain EU trade protections, while Conservatives push for complete removal. Whichever Brexit path is taken could affect the economy of the U.K., and in turn Europe and the rest of the world. Britain does a lot of trade with the U.S., so a downturn in its economy would hurt U.S. exports.
Reference: Refinery29