minibond are to all intents and purposes financial securities, specifically bonds, that a non-listed company may decide to issue.
A bond (ie a bond) is a credit instrument that an entity (therefore also a company) can issue to find funding by practically selling its debt and thus raising liquidity.
With the minibond system, even a non-listed company can benefit from this financing mechanism.
In fact, minibonds are usually issued by SMEs that are not listed on the stock exchange, but which want to open up to the capital market.