Back Stop

What Does Back Stop Mean?
The act of providing last-resort support or security in a securities offering for the unsubscribed portion of shares. A company will try and raise capital through an issuance and to guarantee the amount received through the issue, the company will get a back stop from an underwriter or major shareholder to buy any of the unsubscribed shares.

Investopedia explains Back Stop
For example, in a rights offering you might hear "ABC Company will provide a 100% back stop of up to $100 million for any un-subscribed portion of the XYZ Company rights offering." If XYZ is trying to raise $200 million but only raises $100 million through investors then ABC Company will purchase the remainder.