This is a client on top of NASA's Global Image Browse Service (GIBS), like most of NASA EOSDIS' software and Earth observation data, it is free to use and build with. [1] [2]
Both Worldview and GIBS are open source. [3] [4]
Finally, if you're interested in this, also check out https://search.earthdata.nasa.gov, which is a search client on top of NASA's metadata holdings index (CMR). It's not quite as beginner friendly -- it expects that you're somewhat familiar with remote sensing products and geospatial searching -- but it's a good front end into EOSDIS' data.
[1] https://earthdata.nasa.gov/about/science-system-description/...
[2] https://wiki.earthdata.nasa.gov/display/GIBS/GIBS+API+for+De...