Model Mode is a building tool which allows you to place models on your property and edit them in other ways. It is automatically activated when the Thinking Hat is equipped on a Property.
While in Model Mode, players can take models out of their backpacks and place them, move models that are already on the property, use Behaviors, enter Brick Mode, name and describe models, and put models away. By doing all of these things, a player can organize their property in any way they like.
Players could use Model Mode in "Showcases" during a portion of Beta Testing. These Showcases could be edited by anyone. There is some evidence that suggests these may have been scheduled to return in a future update.