To over-simplify, an inode is an entry in the filesystem's database. An inode can represent a directory or a file, among other things. If you have run out of inodes, then you likely have too many files on your volume. If you can, delete some files or move them to another volume. Another option might be to tar or compress a bunch of files in to one file.
See http://unix.stackexchange.com/questions/26598/how-can-i-increase-the-number-of-inodes-in-an-ext4-filesystemhttps://unix.stackexchange.com/questions/26598/how-can-i-increase-the-number-of-inodes-in-an-ext4-filesystem
