A transaction is when you send money to someone else. When you create a transaction you send a message to the network that some of your money now belongs to somebody else. The blockchain records this message.
For example, Alice could send one ZEN to her friend Bob. Alice will send a message to everyone on the network that she just sent Bob one ZEN.
When Bob wants to spend that one ZEN, he will create a new message to the network. This message will say: “I received one ZEN from Alice and I am sending that one ZEN to someone else now”.
This is how money is sent from one person to another without the help of a central third party.
In our Technology section, we talk more about transactions.