A certificate attached to a public document by the country that issued it, which under the Hague Apostille Convention makes it acceptable in every other member country without further steps.
The catch is worth stating plainly: do not assume an apostille is what you need. Where the receiving country does not accept one, the route is the older and slower path - authentication by the Ministry of Foreign Affairs, then legalisation at that country’s embassy or high commission in Accra.
Ask the receiving institution which of the two they want, in writing, before you pay anyone. Getting this wrong costs weeks.