What is Section 702 of FISA, anyway?

President Donald Trump threw Twitter, and members of Congress, into a tizzy Thursday morning when he tweeted a back-and-forth with himself about Section 702 of the Foreign Intelligence Surveillance Act. The reauthorization for 702 was up for a vote in the House just hours later -- it passed, and now goes to the Senate for consideration -- but the President's tweets arguably brought a little understood surveillance tool to the forefront of public discourse.