Pseudonymisation is the processing of personal data where additional information is needed to connect the data to a person. Pseudonymised data can still be used to single individuals out and combine their data from different records.
Pseudonymisation may involve replacing names or other identifiers which are easily attributed to individuals with, for example, a reference number. Whilst you can tie that reference number back to the individual if you have access to the relevant information, you put technical and organisational measures in place to ensure that this additional information is held separately.
Pseudonymisation is effectively only a security measure. It does not change the status of the data as personal data. Pseudonymised personal data remains personal data and within the scope of GDPR.