Erect means standing straight up or upright. When you erect a flagpole, you set it up so it stands vertically. Buildings, monuments, and statues are erected when they're constructed and raised into their upright positions. The ancient Egyptians erected massive obelisks that still stand thousands of years later.

The word describes both the action of raising something upright and the state of being upright. A soldier standing at attention holds an erect posture, with their back straight and shoulders back. A prairie dog sits erect on its hind legs to watch for predators. When your teacher reminds you to sit up straight, they're asking you to sit in a more erect position.