A join may be used to fetch values from a table other than the base table of a BC. Now if the join returns more than a single record then the data will not be displayed in the UI. Join constraint enables us to filter the records returned from a join.
For eg we have a BC whose base table is S_PARTY and the field is joined to S_PARTY_PER (Explicit join) and the join return more than one records. We can create a join constrain in which we can specify the Destination Column which will be a column in the joined table and in the Value field we can right an expression that would be compared against the destination column and the join will now return records that matches the constraint.
