An eclipse of the sun happens whenever the moon gets between the Earth and the sun, so that the moon blocks your view of the sun. Depending on where you are standing on the Earth, you might see the sun completely blocked (as in these pictures) or only partly blocked, or you might not see it at all.
The moon passes between the Earth and the sun once every month, at the new moon, but most of the time the moon is either too high up or too low to block our view of the sun. About twice a year, the moon does get in the way of the sun, causing an eclipse. But even then, you can only see the eclipse from some places on Earth; the area where you can see a total eclipse is about 100 miles wide. Eclipses move around the earth, so every year people in different places see them.
During the eclipse, the sky gets dark, as if it were dusk. You can see thestars (but don't look directly at the sun unless you have special glasses). Birdsgo roost in the trees, thinking it's bedtime. A total solar eclipse only lasts about four minutes; then the moon moves slowly out of the way of the sun. It gets light again, and the birds wake up.