Translation:Eleven cars are moving in front of the bus.
Előtt comes immediately after the thing that it applies to. Since you see A busz előtt in the sentence, you know it means "in front of the bus."
The whole sentence could be arranged in several word-orders, but if you want the meaning to be "in front of the bus," then a busz előtt has to stay glued together as one unit.
If something was happening "in front of eleven cars" then you would have tizenegy autó előtt somewhere in the sentence instead.
A busz tizenegy autó előtt halad would be one way to translate your sentence, "The bus moves in front of eleven cars."
It is in the lesson tips.
"In English, nouns that follow a number are in the plural, but in Hungarian, they are in the singular."
When a noun has a number in it, that noun will remain in its singular form, and its verb will be in the 3rd person singular form, not the 3rd person plural form.