API stands for Application Programming Interface which allows you to deal with the server without using an actual Graphic user interface of it. It is a set of programming instructions, protocols and standards for accessing the web-based software application. It is a software-to-software interface, not the one created and designed for users.
For example, imagine that you are booking a flight. You open a site of the airline of your choice to check availability and a cost of the flight on a chosen period. For that you will have to access airline's database. For you that would be a click of a mouse and a couple of seconds of browsing. Under covers there is a great work happening to get you these data. The site using API reaches the database to get the information you require and then post it to the site interface you are looking at.
Much more operations are performed using API. But since all of that is happening behind the curtains, it may seem less obvious and more difficult than it is.

API is used for building new software that uses data or works jointly with the chosen program (which API you are using). In a way that code allows two different applications (DSX and your trading robot, for example) to communicate with each other without going through the usual interface. Thus, your robot can analyse the order book at DSX and decide whether you should buy or sell some currency.

The full API documentation of DSX you can find by that link 

When to use API?
Our clients are using API if building robots or accessing their accounts remotely through self-developed terminals.
Where to create API keys?
API keys (private and public) can be created in a personal account at DSX. Please open Settings section for that.

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