In their simplest form, guitar pedals (and put more broadly, effects pedals) are small metal enclosures housing circuitry that change the sound of your instrument, be it an electric guitar, acoustic guitar, bass, synth or anything that has a quarter-inch jack output. Pedals are usually placed on the floor as part of a chain that you active with your foot, hence the slang term 'stompbox'.
A pedal can drive, distort, modulate, or reverberate your guitar tone and do plenty more depending on the design. There’s an infinite combination of effects to explore to add creative, unique dimensions to your playing or simply enhance your overall sound.