This is the 8567th most frequent Russian word.
общее
"Общее" means "common" or "general."
In this sentence, "общее" is used as a noun to mean 'common things' or 'similar traits' between people.
У нас много общего.
We have a lot in common.
Here, "общее" acts as an adjective, describing 'opinion' as being collective or shared by all.
Это общее мнение группы.
This is the general opinion of the group.
In this example, "общее" modifies 'cause' to signify it is shared or mutual among participants.