OpenStreetMap (OSM) is an open-source (open data) project. OpenStreetMap is a collaborative mapping platform that allows users from around the world to contribute, edit, and use geographical data. The data and software behind OSM are open-source, which means they are freely available for anyone to view, use, modify, and distribute under open licenses.
The data contributed to OpenStreetMap islicensed under the Open Database License (ODbL). This license allows for the free use of the data as long as proper attribution is given and any derivative works are also made available under the same open license.
I got addicted to using and contributing on OSM daily and enjoy spending my time improving the map. In fact a lot of closed source maps such as Google Maps and Apple Maps pull from some of the OSM data, so everyone gets to benefit from contributions.
In case you're looking into this out of curiosity, check out the Beginner's Guide and try to verify that the data around your neighborhood is correct and maybe add a point of interest (PoI) or a street name or two. Beware, it gets addictive quite fast.
OSM is also used for humanitarian use thanks to the HOT tasking platform. For example the majority of relief effort in Turkey's February earthquake, Sri Lanka flooding, and the recent Marocco earthquake. Mapping can literally help save lives. It's fun and easy too!