A bridge round provides additional capital between two priced rounds, usually to give the company more time to hit the milestone the next priced round requires. Existing investors are the most common bridge participants because they have the strongest incentive to protect prior investment.
Bridges are usually structured as SAFEs or convertible notes with a discount and a cap, designed to convert into the next priced round. A 'bridge to nowhere', a bridge with no clear next round in sight, is a danger sign for the company.