Indoor plants usually consist of the planter itself (with a hole in the bottom to allow drainage) and a saucer to hold any excess water from the plant. But plants occasionally need to be moved around, and dirt can sometimes leak out of the bottom of the hole in the planter. You can avoid this by covering the hole on the bottom of the planter with a standard coffee filter.
This tip comes from This Old House's website. The coffee filter will allow water to pass through without dragging the soil out too. You don't even have to use a new filter -- a used one with the coffee grounds still in it works just as well. Eventually, the coffee filter will biodegrade and need to be replaced, but at that point the plant's roots should have developed enough to hold the soil tightly and keep it from leaking out of the pot.
10 Uses for Coffee Filters: 7. Make Houseplants Less Messy [This Old House]