Point wand at target
Erects a structure
- " [...] "Erecto!" she added, pointing her wand at the misshapen canvas, which in one fluid motion rose into the air and settled, fully constructed, on to the ground before Harry, out of whose startled hands a tent peg soared, to land with a final thud at the end of a guy rope."
Erecto'' is a spell that straightens out the target object and sets it up.
Hermione Granger, Ron Weasley, and Harry Potter used this spell frequently through out the 1997-1998 school year to set up a tent for themselves while hunting Lord Voldemort's Horcruxes during the height of the Second Wizarding War.
Behind the scenes
Erecting a tent is the only known use in the series, but it could presumably be used to erect any structure given that the required materials were present.
Erectum is past participle of erigere, a Latin verb meaning "to erect".