A model is a bunch of brick which are grouped together. They can be anything from small houses to entire trains, planes, cars, even complete maps, like cities or mountains! There are over a million models to pick through, see if you can find something cool through the toolbox.
Making your own models
Creating your own model is pretty simple. First, build whatever you want to make into a model. Here, I build some bricks that say "Hi!"
Once your model is built, drag a box around ALL of the bricks that you want to be part of the model. Make sure you have them all selected.
Once you have them all selected, right-click and select "Group" from the drop-down menu. A blue box will appear around the entire collection of bricks. This shows you that the bricks are now part of a model!
Now that you've made your model, all that's left is to upload your newly created item. To do this, select everything you wish to upload (in the explorer pane) and then right click on a selected item. After this click 'Save to Roblox'.
A window will pop up asking you what you want to do. Since you want to save a new model, hit Create.
Enter a name and a description of your model. The name should be simple. The description is where you can write something long about what the model is, how it works, how to use it, stuff like that. If you want people to be able to use your model, mark the checkbox named "Publish for free public use"
Hit 'Finish', and your model will appear on the Roblox website!