BLS is the Bureau of Labor Statistics. It's a source of official employment information and statistics, for example information about the unemployment rate or projected job growth rates for various job titles.
The BLS API is a set of developer resources that "gives the public access to economic data from all BLS programs." BLS provides some documentation to get started using their API, and this how-to guide will walk you through the steps needed to get up and running with the BLS API.