OpenStreetMap ID
The identification number of the corresponding node, way, or relation in the OpenStreetMap database.
OpenStreetMap stores everything as one of three elements. A node is a single coordinate, a way is an ordered list of nodes forming a line or an outline, and a relation groups elements into something larger such as a multipolygon. This field holds the number of the element that matches the record.
The number on its own is ambiguous, because each of the three kinds counts from one. A way numbered 12345 and a node numbered 12345 are different objects, so the kind has to be stored with the number.
A way id is a poor long-term key for a building. Redrawing the outline can replace the way and issue a new number, leaving the reference pointing at nothing. An official register code survives that.