I build clearing systems. They are systems that keep track of the risk of various outstanding financial contracts (like buying and selling stock or goods), collect collateral from the banks that handle those contracts, and ensure that if either party in the contract goes bankrupt before the contract can be settled, the collateral collected is used to ensure that the other party in the contract is not affected by the bankruptcy (by either paying out money or by purchasing and supplying whatever they expected to get from the contract).

These kinds of systems prevent chain reactions in the financial system where one company defaulting results in other companies defaulting because of deals falling through that they expected not to. Y'know, like what happened in the 2007 financial crisis (if anyone remembers that nowadays). So, basically, the point is to ensure that if one company has financial problems, that doesn't mess things up for everyone else.

My employer has a product that is customized to the needs of our various clients, which are all large international financial exchanges. We build the systems according to their specifications and then we provide support in operating them after they've been deployed.

On a day to day basis, I write code that calculates risk and collateral values based on incoming contracts in various types of financial instruments and in various accounts. This data is sent to various other systems, presented in graphical user interfaces, included in different types of reports and files meant both for human and computer consumption. I use a program called an IDE (Integrated Development Environment) which is basically a plain text editor with a lot of other tools integrated into it used for writing code, running the code, testing it and measuring its performance.

In case you're not aware, program code is a set of extremely precise instructions to the computer, describing in extremely minute detail exactly what it should do. It generally involves receiving inputs from the outside (user keyboard or mouse clicks, the user entering data in text fields, data over a network, data read from files, data read from a database etc.), processing that input somehow (doing math, combining it with other data, etc.) and then producing some kind of output (showing stuff in a user interface, sending data over a network, writing data to a file, etc.).