A group of (one or more) blocks connected together forms a model.
Tapping a selected block in the world editor will select the whole model.
Purpose of models
Models are a way of organising groups of blocks. Instead of dealing with individual blocks in your world, you can combine them them into complex structures such as cars and buildings. Selecting a model allows for all of its blocks to be copied or moved around together.
Some actions can be configured to affect a single block or the whole model.
Saving a model
Models can be saved by pressing the save button in the world editor. Saved models appear in the Models tab in the Build Panel and in the Models section of your user profile. You can place models in your worlds if you have enough blocks to build it.
How to build a model
Placed blocks will join together automatically if possible. Joined blocks form a model. If some blocks in your model aren't touching, you can use a trigger cube to connect them.
This vehicle is a single model: