In music, portamento is a gradual slide from one note to another. It is very similar to a glissando, but a glissando is deliberately written in the music by the composer and may be a long slide between two or three octaves or more. A portamento is a much shorter slide, usually between two notes which are quite close. Opera singers often used to slide from one note of a tune to another instead of singing each note separately and clearly. The habit of putting in portamento between notes spread to other instruments as well. When listening to recordings of violin playing from the early 20th century we can hear that the players used a lot of portamento.
Over the last half century portamento has gone out of fashion and singers and instrumentalists are taught not to slide from one note to another. However, there are some places, especially in opera, where it can be effective so long as it is not done all the time. It is a matter of taste.