A customs bond or guarantee is a promise, usually backed by a bank or insurance company, that you will pay duties, taxes or penalties you owe. Some countries require this only for specific situations, such as goods moving in transit to another country, temporary imports that will be re-exported, or traders new to the system; others require it more broadly.
A guarantee ties up capital or a credit line, so understand the amount required and how it is released before you commit to a shipment that depends on one. Ask specifically whether your situation (first-time importer, transit shipment, bonded warehouse use) triggers a bond requirement in your country.