What are the letters next to text and email on Android 5.0.1?

[JPScotland]

Letters next to text and email on android 5.0.1

Hi all, I have an HTC One M8 running 5.0.1 and would like to either edit or remove the letters and coloured circles next to each text and email message.

Ideally I'd like to prevent them returning.

Can this be done, if so how can I do this and what else would be impacted (negatively) by doing this?

Thanks for you help.