Anonymised information is information which does not relate to an identified or identifiable person or to personal data rendered anonymous in such a way that the data subject is no longer identifiable.
Anonymisation is the process of removing personal identifiers, both direct and indirect, that may lead to an individual being identified.
An individual may be indirectly identifiable when certain information is linked together with other sources of information, including, their place of work, job title, salary, their postcode or even the fact that they have a particular diagnosis or condition.
Once data is anonymised and individuals are no longer identifiable, the data will not fall within the scope of the GDPR and it becomes easier to use.
One issue with anonymised data is that once supporting data is removed from the data set, it may not be fit for the purpose of the required analysis or processing.
Further information is available here: https://ico.org.uk/media/1061/anonymisation-code.pdf