Light is an important mechanic in minecraft. It affects visibility, the way mobs spawn and react to a player, plant growth and plant survival. Light is emitted by the sun, moon and some blocks, like torches, glowstone and active furnaces.
Light in the Overworld will change depending on the time of the day. During the day the sun will emit a light of level 15, which is the brighest anything can be. At night, the moon will emit a light of level 4, which means other light sources are often needed to be able to see the world around you well. The Nether and the End have no day night cycle, but they have a constant dim light level, which is bright enough to be able to see without having to place extra light sources.
As mentioned, the light levels affect the way mobs spawn and react to players and it affects the growth and survival of plants.
All opaque blocks will block light, so it’s often useful to use non-opaque blocks, like glass, to let light into your house without having to use torches.
Non-opaque blocks will decrease the light intensity by 1, though this is the same as air. However, water and ice will decrease light intensity by 2, which could sometimes make for some great lighting effects. Note that leaves are only opaque when fast graphics mode has been turned on, they’re transparent in fancy graphics mode. Also, despite being partially transparent, farmland, slabs and stairs completely block light, despite the small gaps created when they’re surrounded by other blocks.
In the video options menu, you have the choice to turn smooth lighting on or off. Smooth lighting creates better shadows and blends light better, which makes everything generally look better and less flat.
Another option is brightness, which will obviously change the brightness of your screen. If you set it to the lowest setting, nighttime will be very dark and moving around will be almost impossible as it’s very difficult to see anything. Turning it all the way to maximum will let you see pretty much everything at night, even most far away details.