HDFS file system namespace
It means that the NameNode inserts the file name into the file system tree and allocates a data block for it. This actually happens when you are trying to put the data into HDFS.
Yes it is possible to have any number of data directories. Here is what you have to set in hdfs-site.xml in the conf folder.
<property> <name>dfs.data.dir</name> <value>path to data dir 1,path to data dir 2 etc</value> </property>