When a company, partnership or limited liability partnership is registered at Companies House it will be issued with a certificate of incorporation. In some countries this may be referred to as a licence, but in the UK it is simply confirmation that a legal structure has been formed.
It is a formal certificate confirming the entity has been registered at Companies House. They are normally printed on cream or white coloured card but most certificates are now issued digitally as a PDF document.
In years gone by, a business used to display their certificate often framing them and mounting them on the wall. This is less common now as most registrations can be easily checked online.