What is the importance of "pre" in nodejs version v0.6.21-pre?

Some versions are having "-pre" at the end of version number and some are not having. What is the importance of "-pre"?

It means the binary was built from a development or "preview" commit.

From src/node_version.h:

#if NODE_VERSION_IS_RELEASE
# define NODE_VERSION_STRING  NODE_STRINGIFY(NODE_MAJOR_VERSION) "." \
                              NODE_STRINGIFY(NODE_MINOR_VERSION) "." \
                              NODE_STRINGIFY(NODE_PATCH_VERSION)
#else
# define NODE_VERSION_STRING  NODE_STRINGIFY(NODE_MAJOR_VERSION) "." \
                              NODE_STRINGIFY(NODE_MINOR_VERSION) "." \
                              NODE_STRINGIFY(NODE_PATCH_VERSION) "-pre"
#endif

The -pre is removed for releases (ex: 2012.08.03 Version 0.6.21 (maintenance)) and added again with the subsequent version bump (ex: now working on 0.6.22).