Tax code

My salary is 65k and I made around 8k in interests and dividends. I don’t qualify for the self assessment as below 10k, but I am required to contact HM to change my tax code. What does it mean? Who calculates and how do I play my taxis?

Hi sdriusso DRIUSSO,
The banks and building societies submit the untaxed interest details after the tax year end.
You will need to contact HMRC to declare your dividend income:
Income Tax: general enquiries
Once the details are received your record will be reviewed and if you underpaid your tax a calculation will be issued to you. 
Thank you.