An intermediary bank (correspondent bank) is a bank on the route of an international transfer through which the money passes between the payer’s bank and the recipient’s bank when they have no direct relationship.
In plain words
A SWIFT transfer rarely goes directly: one or more intermediaries that hold accounts with each other may stand between the banks. Each of them can deduct its own fee, so the recipient often gets less than was sent and the transfer takes longer. The payment order says who pays the charges: OUR (the sender pays), SHA (shared: the sender pays their own bank, the recipient the rest) or BEN (the recipient pays). To make sure the full amount arrives, the payer can choose OUR, but their bank will then charge more.
Example. You sent USD 1,000 and the recipient sees less on their account: intermediary banks kept part of the amount on the way (the numbers are illustrative). The bank can usually tell you how to track the transfer.