The phenomenon in which the shadow of the Moon falls on Earth's surface. They occur every 6 months, which is twice in an Earth year.
They are only visible at a specific, often remote, location on Earth so humans don't usually see them.
Solar eclipses are dangerous to watch because your pupils get bigger to take in more light when it is dark and since the moon is in between the Earth and the Sun it becomes dark outside during the eclipse. There is still a ring of light visible though since the Sun is so much bigger than the Moon so you will take in too much light, which can burn your eyes.