Most houseplants are tropical plants and require a humid environment to grow well indoors.
A pebble tray is used during the winter to counter the dry indoor air and increase the much needed humidity around your houseplants. It basically consists of flat container that is 2-6 inches deep .  It can be an old cookie sheet, plastic tray or anything you can find that is waterproof.  Spread an even layer of pebbles, gravel or stone about 1/2 diameter or less. Pour water over the pebbles, keeping the water level just below the top of the pebbles. Place your plant on top of the pebble tray.  For optimal results use a tray that is as large as the spread of the plant. The leaves will benefit from the rising, evaporating water  (humidity).  Make sure you replenish the water to keep the tray from drying out.
If you don’t have any pebbles etc. place a brick or wooden block in the middle of a tray and stand your plant in it. This method will also work. The important thing is that you keep the bottom of the pot from sitting in water. Grouping plants in a pebble tray is an effective way to increase humidity around your houseplants.