Yes, Kansas had a special election for a House seat. It was closer than expected. Don't make more of it than there is. Here's what we have seen, historically.
1) In midterm elections, the House swings against the party of the president, barring weird stuff, like 9/11, or impeaching the president for a blowjob. The Democrats did better than expected, and we expect the Democrats to pick up seats in 2018. Foreshadowing? .... Don't expect a massive electoral landslide in 2018 without something like an economic crash, a yugely unpopular war, or something on that scale.
2) Weird things happen in one election (hey, what's pussy-grabber from Celebrity Apprentice doing in the Oval Office?). Political science is about finding patterns, which come from multiple observations so that random noise cancels out. With one election, you can't tell if it is random noise or not.
3) It is April. 2017. A lot can happen between now and November, 2018. The range of possibilities for a midterm swing is pretty large, and while the Democratss are hoping for something like 2010 in reverse, a big terrorist attack could have the opposite effect. The Republicans picked up a few seats in 2002. November 2018 is a long way away.
4) Voters can respond to expectations in weird ways. The special election in Kansas was expected to go GOP because, well, it's fucking Kansas. Combine that strong expectation with high levels of attention and you get weird cross-pressures. Normally, in a non-competitive seat, nobody at all pays any attention to the race because it just isn't competitive. So, the dominant party just wins in a low-profile, low-turnout all-around election. In a special election, with all eyes on the race, well, you ever act differently when you feel like you're being watched? Democrats see polls where they think they might have a sliver of a chance, and they get mobilized, and you get an effect of having been observed which wouldn't be the case had the election been held normally.
The Kansas special election was closer than expected. What does it mean?
It means... I don't know. It is one observation. Don't make too much of one special election.