Side vents, also known as secondary vents, in a volcano allow some of the magma and gases to escape but are not the main vent where the eruption takes place. One of the volcano's most recognized characteristic is the crater, but this hole actually forms after the eruption has taken place.
The eruption primarily goes through the main vent, but there can be various other secondary vents as well where smaller amount of magma escape. The vents act as channels for the magma coming from the chamber to the ground's surface. The different types of volcanoes have different structures and can have many side vents.