An API, which is an acronym for application program interface, is software that lets programmers work with a program, such as to extract information from on online site. Many APIs are available for specific websites. For example, Amazon or eBay APIs allow developers to use their platform to create specialized web stores.
To search Twitter, for example, and find out how many tweets there have been about a particular general counsel, you have to be authorized to use the Twitter API. That step takes a bit of work, but you eventually receive a personalized set of access codes (called keys).
Once you have API access, you can search and retrieve. Using the data returned, you can turn to other software to analyze the frequency, volume, and content of the tweets. In our world that is dense with online information, some facility with APIs will be crucial for those who want to harvest that trove.