A RID lookup is a lookup into a heap table ( A table without any clustered index. Even if we have a non-clustered index on heap table, it can act like a heap table). So in a RID lookup, a table is scanned using RowID (Row by Row that's why the name RID LOOKUP.
Generally Row ID is included in a non-clustered index in order to find the rest of a table's data in the heap table. Since a heap table is a table without a clustered index and rows are not sorted out in any order, it does a entire table scan.
To make sure that table does not do a RID LOOKUP, we can solve this issue by building a clustered index on a table.
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