A little help here

I know a few things about Bitcoin but not technically so as I went on with reading, I came across this topic about script and transactions so I learn and understand a few things about script and output but somethings are still not clear.
I read that script is a set of instructions that determine how to spend a transaction output. That it's like a lock that secure outputs and set rules for spending the output. While output is an amount being sent to an address. Also learnt about two types of output;
Spendable output: Outputs that can be spent and used as input for new transactions.
Unspendable output: outputs that cannot be spent or used as input for new transactions. Which led me to OP RETURN which confused me even more.
- I thought the Blockchain only deals with transactions alone.
- what use do arbitrary data have on the Blockchain and why are they allowed to be stored there? Won't they affe
- understanding OP RETURN is a bit challenging on my own so I'd like a more simpler explanation.

Any further explanation and corrections on this topic will be appreciated I think I got myself confused.Thank you.