bobsFollowers[i] is the
string (person) at index
and if that matches someone in
tinasFollowers, (maybe at index
then you want to add that person (or
string) to the array
mutualFollowers started out as an empty array.
mutualFollowers.push(x) would add whatever is stored in
x to that array.
mutualFollowers.push(bobsFollowers[i]) adds the person from
bobsFollowers at index
i to the array