In Roman mythology, Venus was the goddess of love. From Earth, the lovely planet Venus is the brightest object we can see besides the Sun and the Moon. We know today that Venus itself is not so lovely a place, with clouds of sulfuric acid and a surface hot enough to melt lead. But because Venus is between the Earth and the Sun, every so often, something very special happens. Venus passes directly between our planet and our star. We call this passage a transit.
Imagine yourself as a space alien, looking down on our solar system. In the center is the Sun, dwarfing all else. A few million miles* from the Sun you see some orbiting rocks. Two of them, almost the same size, are the second and third rocks from the Sun. The orbits of these planets are almost (but not quite) circles, and they’re almost (but not quite) in the same plane.
If the planets were exactly in the same plane, it would be as if they were marbles rolling about on a flat plate as they circle the Sun. If that were the case, then every time Venus overtook the Earth in its orbit (something that happens at least once every year) people on Earth would see Venus pass in front of the Sun. But because our orbits are tilted, this perfect passage (or transit) occurs only when conditions are just right.