This is related to the transaction model - it means output number vout of the transaction with ID txid. From the MultiChain white paper:
The... blockchain uses a per-output transactional model, in which every transaction has a set of inputs and a set of outputs. Each input “spends” one output of a previous transaction, with the blockchain ensuring that this output cannot be spent elsewhere. The full history of transactions forms a multiway connected chain... All of the [assets] in a transaction’s inputs flow into that transaction, which are then distributed across its outputs in accordance with the quantities written within. As a result most regular payments require two outputs – one with the intended amount for the recipient, and the other containing “change” which goes back to the sender for use in a subsequent transaction.