This is a specific technique for playing an electric or acoustic guitar that has its roots in the early 1900s. Lots of objects have been used as slides, the most common of which is the neck of a glass bottle, though any glossy metal, glass, or ceramic object will do that can fit over a finger of the fretting hand. The late Duane Allman expressed his preference for empty Coricidin bottles, which led to an entire generation of slide guitar players that bought up lots of Coricidin, only to toss out the contents just to have the bottles Duane endorsed (and perhaps in the hopes of getting just a bit closer to Duane’s fiery slide playing skills).
If you actually want to get technical, the term slide refers to the sliding motion of the glass, metal, or other object from note-to-note, producing a distinctive sound that is unmistakable. On electric guitars, the action is typically set a bit higher so that the slide doesn’t even press down on the fingerboard. Early slide guitar players preferred resonator guitars such as the classic Dobro. See also “Bottleneck”.