Hello,
When our Moon passes between the Earth and the Sun, we call it a solar eclipse. The angular size of the Moon and the Sun in our sky on the Earth are about the same, so the Moon can block out the entire disk of the Sun in a solar eclipse. The angular size of the planet Mercury is quite small as viewed from the Earth, so that Mercury can never entirely block out the Sun's disk for an observer on the surface of the Earth. When an solar system object with a small angular size passes between an observer and an object with a much larger angular size, scientists call this a "transit" instead of an eclipse.
In the past, transits have been used by scientists to learn things about objects in the solar system, such as their orbits and sizes. Transits of Mercury across the Sun helped scientists to check Einstein's theory of relativity. Even though we can now send spacecraft to other planets, transits are still helpful to scientists. For example, by carefully studying the timing of the transits of Phobos and Deimos across the Sun observed by the Mars rovers, scientists improved our knowledge of the orbits of Phobos and Deimos.
Kris