Great question as this isn't a title I was familiar with until I myself joined the company, but this is a common title in Palantir and other consulting companies. It's an engineer role that interfaces with external customers. While we're not a consultancy, some of the work that an FDE will perform is very similar to that of consulting as they will be directly interfacing with external customers.
In B2B businesses, it is usually used for engineers that help integrate client systems to their own systems because when an enterprise is paying millions for a piece of software, they don't want to change their own systems. Work would typically involve writing scripts, interacting with customers understanding their needs and coming up with a bespoke solution that works for them.
