An index node (inode) is a data structure that contains information about a file. It DOES NOT contain the actual file data itself but only the metadata and the pointer(s) to the data blocks that contain the actual file data. It is important to note that an inode DOES NOT contain the file name either. A directory contains filename and inode mappings.
An inode table is created when the filesystem is created. Some filesystems allow for dynamically modifying the size of this table, but ext4 does not. Usually 1% of the total space is allocated for the inode table. Each table entry is an inode and also has fixed size.