Certificates describe a wide variety of academic qualifications. There is big differences between lenght, scope, accreditation and acceptance of certificates.
A popular example for certificates are language certificates such as TOEFL and IELTS that are widely accepted as a proof for English knowledge in an academic context. The GMAT for example is used for admission to graduate management programmes.
Certificates can be completed along with the studies or for example during Summer Schools or trainings.