Generic question about the error "Waiting for service /gazebo/spawn_urdf_model"

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Hello,

I had a generic question about a specific error.

It says Waiting for service /gazebo/spawn_urdf_model and the terminal freezes. This usually happens when I'm trying to launch any robot in Gazebo.

I've always figured out how to go past these errors but never really understood "why" and "when" exactly these errors pop up? What do they mean and what's the general workaround to get past these errors?

TIA