Eclipse (grahan )are times of the year when the path of the Sun and the Moon intersect with the Nodes of the Moon, Rahu and Ketu. A Lunar eclipse (chandra grahan )is during the full Moon and the Solar eclipse is during the new Moon.
What is the significance of an eclipse?
Let's first talk about the basic significance of the Sun, Moon and Rahu and Ketu. The Sun and the Moon are what can be called 'personal' planets because they represent our personality more than any of the other planets.
The Sun is the karaka or signficator of the body and our self-expression, while the Moon is the karaka of the mind (not intellect, which is Mercury) and emotions. The nodes of the Moon, Rahu and Ketu, are called chaya grahas or shadowy planets because they are invisible points in space and not actually planets with orbits around the Sun. They are given the full status of planets in Vedic astrology none the less. Their points in space which are always opposite one another travel through the zodiac in reverse direction in an approximately 19 year cycle.
Because the nodes of the Moon are so mysterious and unusual in nature their influences cause abnormal functioning to whatever they aspect. During the eclipse the nodes of the Moon are aspecting both of the personality planets, the Sun and Moon and are cutting off their normal functioning. They create an opening in our psyche for subtle energies to enter. It's therefore the best to only do spiritual practices during eclipses and not anything in the material world of signficance like having a surgery, or signing an important document. We should take a break and do extra spiritual practices and take advantage of the subtle influences of the eclipse time.