1.
beautiful, pretty
Describes something or someone that is aesthetically pleasing. Often used for scenery, flowers, people (especially women), and objects.
2.
clean, neat, tidy
Describes something free of dirt or mess. Used for rooms, clothes, hands, etc.
きれい is a na-adjective with two common meanings: beautiful/pretty and clean/tidy.
Usage
きれい takes な before a noun (きれいな花, "a pretty flower") and に before a verb (きれいに洗う, "to wash clean"); as a predicate it takes だ/です. Context distinguishes the two senses — "beautiful/pretty" (scenery, people, colors) and "clean/tidy" (rooms, hands). It is normally written in hiragana.
Forms
Sense 1 - beautiful
Sense 2 - clean
Writing
Usually hiragana. Kanji 綺麗 exists but is less common.
Gender note
For people, きれい is typically used for women. For men, かっこいい (cool/handsome) is more common.