Question

what is the function of aquaporin

Answer 1

Aquaporins are "the plumbing system for cells.
Aquaporins selectively conduct water molecules in and out of the cell, while preventing the passage of ions and other solutes. Also known as water channels, aquaporins are integral membrane pore proteins. Some of them, known as aquaglyceroporins, also transport other small uncharged solutes, such as glycerol, CO2, ammonia and urea across the membrane, depending on the size of the pore. However, the water pores are completely impermeable to charged species, such as protons, a property critical for the conservation of the membrane's electrochemical potential.

Answer 2

Sgholap100 is right.

I'd just add that aquaporin faciltates passive transport down a concentration gradient only. Just like plumbing pipe.