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 send Bob one ZEN.
When Bob wants to spend that one ZEN, later on, he will create a new message to the network. This message will say: I received one ZEN from my friend earlier, I want to send it to somebody else now. This is how you send money from one person to another without the help of anybody else.
This is similar to how you get a dollar bill from somebody and hand it to the next person when you want to spend it.
We talk more about transactions in our Technology Section.