DB File scattered read wait event occurs when an oracle process waits for a physical read of multiple oracle blocks from the disk. This wait identifies that a full scan (Full Table Scan or Index fast full scan) is occurring BUT during this read the blocks are read into memory locations that are not physically adjacent to each other, hence the term scattered read.
In simple words, when a SQL Statement causes a full scan, oracle’s shadow process reads up to DB_FILE_MULTIBLOCK_READ_COUNT consecutive blocks at a time and scatters them into buffers in the buffer cache. Since a large no. of blocks have to be read into the buffer cache, server process has to search for a large no. of free/usable blocks in buffer cache which leads to wait included in db file scattered read wait.