A certificate is a document that says something is true. It may be an award that can be given to people. The certificate can be given to people for graduation, for good working skills, or to thank someone who has done good things. Other items, such as patches, badges, lapel pins, watches, and medals can also come with the certificate.
Many other kinds exist. A certificate of authenticity says a thing is real. For example it can say that a painting was made at a particular time by a particular artist and is not a forgery. A certificate of occupancy says a building is fit for use. A certificate of deposit says someone has money in a bank.