On Monday, November 11, 2019, Mercury passed directly in front of the sun, as viewed from Earth. While Mercury passes between Earth and the Sun multiple times each year, Mercury usually tracks above or below the Sun because Mercury's orbit is tilted relative to Earth's. So, the only time Mercury will appear to pass directly in front of the Sun is when it is not only passing between Earth and the Sun but also crossing the Earth's orbital plane at the same time. You can think of it as two circles nested inside of each other, with one tilted relative to the other. In two locations, one on each side, both circles will cross the same line. When both Earth and Mercury are at the points in their orbit where the planes of those orbits intersect, then they will also be aligned with the Sun. That doesn't happen very often, and so it will be another 13 years before Earth and Mercury line up in a way for us to see Mercury pass directly in front of the Sun.
You can watch the full Mercury transit in the YouTube video below.