What is blockchain? Well, at its simplest concpet, it's really not more than a distributed database.
Think of it as a large worldwide computer where everyone can securely access data and execute transactional code. All transactions are stored in blocks of data.
These blocks are made in a way that makes them very hard to manipulate or fake once they are stored to the blockchain. Due to the nature of blockchain, you can say that you it gives you a trustworthy way to store data in scenarios where there are no trust.
This could be financial transactions between anonymous strangers on the internet or the ability to securely store your medical information in a way that can only be accessed by those you allow.
It is also worth mentioning that blockchain is generally not a place to store large amounts of data for each transaction. You would, for example, generally not store images on blockchain, but you might store information to validate if an image is being tampered with or not. Most data stored on the blockchain is focused on transactions and states of objects, rather than the actual objects themselves.