Partycle system's labels error? I'm following this Gleb Alexandrov tutorial :

Blender tutorial: How to Create a City, Covered with Fog

Go to minute 4:52,under the particle system/render tab,you will see that he chosen as Dupli Group "Group.001". Where/when he took/created this label ? I don't have it on my side. He also has 2 others label : g_antenna and group. I imagine that g_antenna is related to the mesh of the antenna that he got somewhere,but I'm not able to understand where,and the other label,that's group,I don't know what is this. 

I add two screens. The first is what I see on my side and the other one is what I see on the particle system/render tab of the Gleb Tutorial.

1) 



2)
Mate, Gleb created roof details meshes (antennas, chimneys and stuff) himself and grouped them to use in the particle system. You won't find them in your .blend file. You can try to find similar objects online, but if I were you, I'd just create them myself. It would be nice exercise for a beginner.

To group objects, you need to select them and press Ctrl+G. On the left bar (under T key) group name field will appear. You can leave the name as Group or change it to your liking. Highlight of grouped objects should turn green. Check this image to see an example:



The group name will appear in particle settings.