In philosophy it is what makes you a functionalist compared to a materialist.

Materialists think that everything is made our of matter, even our consciousness.
Functionalists go even further thinking that even if everything is made out of matter, even our consciousness, function is the core of everything.

Thus, in this case, what makes you an "individual" is not actually "your" brain, nor "your" cells, but rather the functionning of the connections in your brain which then creates "you".
In other words, it doesn't really matter what we are made of, "we" are just an illusion that lives between two cells.