Every second, transactions take place between parties everywhere around the world. In the context of blockchain, you will most likely think of transactions like processing a payment between two parties, placing an order on an online store, or keeping track of accounts. But if you think outside the box, there are so many more kinds of transactions that you, as an Oracle developer or customer, can imagine. For example, the registration of vehicles, tracking diamonds around the world, keeping medical records, sharing documents, and getting access to your favorite online service all involve transactions. In most cases, each participant keeps track of these transactions by recording and securing them in a ledger (for example, a database, spreadsheet, or computerized record tracking system), thus holding their version of the truth. As you can understand, this is a perfect recipe for human error, or possibly even fraud when using multiple ledgers.
Transactions can definitely be complex...