This is the abbreviation of Italian glissando, which means portamento. This mark will be marked between two notes, so when playing or singing, you can quickly slide from the previous note to the later note. For example, when playing the piano, slide your fingers across the keyboard, quickly starting from the starting note through all the intermediate notes and then to the ending note. If it is some stringed instruments, you can slide from the starting sound to the ending sound at a continuous frequency, which means it sounds more delicate, rather than the grain-by-grain feeling of keyboard instruments. So the specifics also depend on the example. If you have an example, I can analyze it based on the example.