In the City, you get hired based on a "total comp" package that includes salary and a practically guaranteed bonus, that can be as much as your base salary or even more.
It's the UK term for Wall Street. It doesn't have as much of the geographic connotation these days - working in Canary Wharf or a Mayfair hedge fund can all come under the phrase of 'working the in the city'
Unless the devs in question are building trading algos or other front-line work the bonus isn't often much more than other industries. The idea you're going to get a 100%+ base salary bonus when you work building order resolutions for a T2 investment bank in the City is laughable.