It will begin syncing as it hears about higher blocks from nearby nodes.
It will verify transactions in the received blocks. This process includes updating its local copy of the state - account balances and contract storage. This means catching up to the rest of the network in all respects.
It will also be able to sign and send transactions. However, it may not receive confirmation nearly as fast as the user expects. This is because the sent transactions would be mined and accepted by the rest of the network at a higher block number because the node is behind. When it catches up, it will discover that the confirmation has been known to other nodes for possibly a long time.
To understand how it can sign and send transactions (propose them to the network) when its behind, it might help to keep in mind that nodes never really know what the highest block in existence actually is. They can never self-assess that they are 100% synced up.
Hope it helps.