Ear Tips are used in earphones only and it is worn over the earphone stem aka nozzle which helps our ears to get the secure and tight fit. Different kind of earphones offers different fitment, isolation and sound too. The sound signature sometimes changes drastically from different ear tips to ear tips and one should experiment with different ear tips to get the desired sound they want.
Ear Tips comes in 3 sizes:
If your ear canals are very small so you should try the small ones, if your ear canals are in average size so medium size will fit, if your ear canals are large in size so large size will fit.
Please note, different manufacturer's ear tips may sound different on the same earphones you are using so always experiment with ear tips to get the desired sound character.