Each square box on this diagram represents a transaction being sent.
Each time a transaction is sent, 2 transactions are validated as part of the process (see the 2 dotted arrow lines extending from each transaction to 2 other transactions).
The grey boxes on the right represent new (non-validated) transactions.
The red boxes represent transactions that have been validated a few times.
The green boxes represent transactions that have been validated a sufficient number of times, in order to be accepted as “confirmed” by its recipient.