Altar tools are the tools that are needed to practice and cast spells and rituals. Altar tools can be anything from altar bells, candles, incense, offering bowls, athames, etc. Basically, any tool used for a ritual or spell is considered to be an altar tool.
Altar tools are meant to be kept on the altar when not in used, as this helps to absorb the energy from the altar itself, making the tools much more powerful and potent for ritual and spell work. Altar tools should be kept on an altar cloth, as this helps to keep them clean, and also contains the energy within them when not in use.
Almost anything can be considered an altar tool, but the most popular and widely used ones are athames, altar bells, offering bowls, altar tiles, chalices, and ritual boxes.