What is blockchain in simple words?

Blockchain is the system where blocks are distributed on many systems and connected with help of an unique id. It makes a chain, that's why we called it blockchain.

First, let me describe the terms.

Block: It stores the data. Each block has its own id which is unique within blockchain system. Unique id has been created using previous block's id and it's data as input using SSA256 hashing algorithm.

Nodes: These are the distributed systems which contains some blocks. It helps verifying the data. If we talk about the bitcoin blockchain system, the nodes are called bitcoin miners. Nodes are anonymous for security purposes.

How blockchain is secure?

Block's datas are immutable. If you try to change any block's data, all the next blocks in chain will recognise that you're trying to change something. It's because, on different input, we get the different hash id, and when hash id will change next block can recognise the change. That is why it's more secure than traditional database.

Can I create my own blockchain system?

Yes, you can create your own blockchain system, only have to write some lines of code. You must take a look at Smart Contract.