Encryption: first, all connections in Tor use TLS link encryption, so observers can’t look inside to see which circuit a given cell is intended for.
A cell in Tor is a bit like a packet over the internet. It’s a somewhat self-contained block 500ish bytes long. There are many kinds of cells for many usages, but the one referenced here are mostly the RELAY_ family of cells, which can contain things such as to which website you want the connection to go to, or the content you are exchanging with that destination.