what is the meaning of namespace and metadata which were used in hdfs(namenode)

According to 'Hadoop The definitive guide' - "The NameNode manages the filesystem namespace. It maintains the filesystem tree and the metadata for all the files and directories in the tree."

Essentially, Namespace means a container. In this context it means the file name grouping or hierarchy structure.

Metadata contains things like the owners of files, permission bits, block location, size etc.