According to the United States Election Project, about 47.5% of eligible voters participated in the 2018 midterm election. That may not sound high, but it is the highest percentage of voters voting in a midterm election since 1970 (nearly five decades). And it was significantly higher than in 2010 (41%) or 2014 (36.7%).
The old adage says that a large turnout tends to favor Democrats, and that was certainly true in 2018. The turnout was large enough to overcome the gerrymandering Republicans had built into the system in many states -- and large enough to give Democrats a significant majority in the House of Representatives.
Of the voters who participated, about 51.4% voted for Democrats and 46.9% voted for Republicans.