Let’s start with a short video:
Who has influenced you in the past months?
This might give you an idea of what social proof is: “People will do things that they see other people are doing. For example, in one experiment, one or more confederates would look up into the sky; bystanders would then look up into the sky to see what they were seeing. At one point this experiment aborted, as so many people were looking up that they stopped traffic.” This is the description you find on Wikipedia. This interpretation of social proof focusses on action, but in the same way people might influence each others thinking.