sub-editing a news article

proofreading – check spelling, grammar, sentence & paragraph structure
sense and style – ensure mode of address and language code is in keeping with paper, as well as ensuring enough context is present. Trimming article if necessary – either for quantity or quality reasons.
editing quotes & readers’ comments – shortening and formatting where necessary
writing headlines - active voice, awareness of tense, some of 5ws, snappy
writing stand firsts – first few lines of an article, usually in bold, elaborates on headline

You will need to consider:
checking pictures and captions –  accuracy and suitability
legal and regulatory compliance – IPSO, libel, media law
meeting the brief

The sub-editors role is to say this is interesting but where is the information from, who is your source? They will question the writer and pressure them to provide evidence as well as question how they obtained the information if it was done in an ethical way.