Openstreetmap node IDs were implemented as a 32-bit variable. Since 9 February 2013 the variable container is too small to store unique IDs. Therefore users and developers are invited to update their software and use tools which are capable to store 64 bit identifiers.
Currently the project counts more than 2147438647 node identifiers, which are too much for a 32-bit integer variable.
Now more than 2 billion nodes, single points with latitude and longitude information are used to describe certain parts of the earth. The for the basis for geographic objects like the path of a road and other geographic features.
The majority of tools related to the OSM project was already updated to prevent problems with this limitation. Currently only a few tools left have problems with OpenStreetMap. The node IDs are continually increasing. If you want to know how many nodes IDs the project currently has you can have a look under textual.ru/64. There are already more than 6.5 million IDs longer than 32bits.