A metronome is a device that provides a steady ticking sound along with a visual indicator of the tick. Metronomes work in Beats Per Minute, and can be set at particular speeds–usually between 40 and 208. There have been a number of scientists, musicians, and inventors who have worked on various devices over the years. The most common of the mechanical metronomes was developed by Johan Maelzel, who worked on it with his friend Ludwig van Beethoven in 1815.
Metronomes are an indispensable practice tool for all pianists beyond the first year of lessons. Metronomes help pianists develop an internal pulse. They allow students to control their practice tempos and refrain from speeding up too quickly. Likewise, metronomes push students to go faster than gradually and work on bringing out the full energy of a piece.
Metronomes come in a variety of forms these days. There is a free app called Metrotimer that works well on android and iOS devices. There are also electric standalone metronomes like this one or this one, or mechanical metronomes like this one. My preference is for a mechanical metronome. Good ones can last twenty years, don’t need batteries or electricity, and keep piano practice free of smart-phone distractions. On the other hand, they are more expensive. Any variety of metronome will get the job done.