O termo backstop é usado em Finanças e se refere a “o ato de fornecer suporte ou segurança de último recurso em uma oferta de valores mobiliários para a parcela não subscrita de ações. Uma empresa tentará obter o capital através de uma emissão e, para garantir o valor recebido através de emissão, a empresa receberá um backstop de um subscritor ou acionista principal para comprar qualquer uma das ações não subscritas” (INVESTOPEDIA, [20--?], tradução nossa).

https://www.researchgate.net/publication/322848420_O_backsto...

A backstop purchaser, also called a standby purchaser, is an entity that agrees to buy all the remaining, unsubscribed securities from a publicly traded company’s rights offering, or issue.

When called upon, a backstop purchaser functions as a form of insurance. In exchange for a fee, they provide security, guaranteeing that all of the newly issued shares will be purchased. In other words, the company behind the issue can be sure of fulfilling its fundraising requirements, regardless of open market activity.

KEY TAKEAWAYS
A backstop purchaser is an entity that agrees to purchase all the remaining, unsubscribed securities from a rights offering.
In exchange for a fee, they guarantee companies that their capital requirements will be met.
Backstop purchasers are usually called on after other underwriting parties have failed to sell all of the shares at a discount to the public.
They don’t come cheap and tend to charge a premium for the risks they take on.

https://www.investopedia.com/terms/b/backstop_purchaser.asp