An extent is a contigious fragment of the file, and all extents link to each other, so the filesystem doesn’t have to keep track of all data blocks at once, i.e., a filename only links to the file’s first extent. This is more efficient and saves space in the file inode (place where the disk keeps metadata and stuff). Before ext4 (which is extent-based), ext2 and 3 stores links to the first few data blocks and then creates an “indirect block” to keep track of the rest of the block if necessary.