Basically, a lava tube is formed when a underground lava flow stops flowing, leaving behind a long tubular cave.

A lava tube is a natural conduit formed by flowing lava which moves beneath the hardened surface of a lava flow. Tubes can be draining lava from a volcano during an eruption, or can be extinct, meaning the lava flow has ceased and the rock has cooled and left a long cave.

 

Lava tubes are a type of lava cave formed when a low-viscosity lava flow develops a continuous and hard crust, which thickens and forms a roof above the still-flowing lava stream. Tubes form in one of two ways: by the crusting over of lava channels, and from pāhoehoe flows where the lava is moving under the surface.1

Source

This is what the lava tubes at Mammoth Cave (Utah) look like,

Basically, a lava tube is formed when a underground lava flow stops flowing, leaving behind a long tubular cave.

A lava tube is a natural conduit formed by flowing lava which moves beneath the hardened surface of a lava flow. Tubes can be draining lava from a volcano during an eruption, or can be extinct, meaning the lava flow has ceased and the rock has cooled and left a long cave.

Lava tubes are a type of lava cave formed when a low-viscosity lava flow develops a continuous and hard crust, which thickens and forms a roof above the still-flowing lava stream. Tubes form in one of two ways: by the crusting over of lava channels, and from pāhoehoe flows where the lava is moving under the surface.1

Source

This is what the lava tubes at Mammoth Cave (Utah) look like,