Basically they're what a bank has, to help mitigate losses and loan defaults. If it were a person, it'd be like a car or a house, something physically "holding" a value that you can tap into (via equity or sale) in case of a financial emergency or necessity.

Hope this helps. I've attached a link to a paper written by the Fed Branch in Philadelphia for a better idea. Have a great week.

An alternative to Cristiana's post. This is also used at the ECB and EBA. Both terms are often used sometimes in the same sentence.
"Unencumbered" however is the term rather than "not pledged". In legal terms when you write a loan contract requiring collateral, it is very important that the collateral pledged is unencumbered, i.e. not already pledged to somebody else. Similarly when buying a house, you need to know that it is free from encumbrances.

https://www.google.co.uk/#q="pledged assets"

Guidelines on disclosure of encumbered and ...
https://www.eba.europa.eu/.../c65a7f66-9fa5-435b...
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27 giu 2014 - Disclosure requirements in Council Directive 86/635 set out the amount of assets pledged, assets available to be pledged and a breakdown by ...