Star lifting is the practice of harvesting materials from a star. Although stars are mostly hydrogen and helium, there is a large number of other materials present, practically every stable element on the periodic table.
This is similar to Gas Giant Lifting, a process where the material is taken from a gas giant.
This process is typically never done by humans because of the large energy requirement and that it's cheaper to simply mine asteroids or planets. Civilizations with a smaller amount of territory may lift stars because they only have so much material under their control that isn't in stars.
In order to carry out such a process, a large ring is constructed around the star. Then the ring needs to emit an extremely powerful magnetic field to attract stellar mass towards it. The amount of energy is absurd, requiring almost half of the energy supplied from a typical Dyson sphere to do. However, gigatons of metals can be extracted this way, making it worth it to civilizations without much material.
Star lifting is also an economically impossible way to gather materials. The amount of energy required coupled with the requirement to build a ring makes doing this barely profitable. Large amounts of materials are gained, but selling them back yields little return on investment.
Some times, a star is lifted to increase its life span. That is especially useful when making Dyson spheres. Most of the time the stars are turned into red dwarfs to increase their life spans one thousand fold.