The document describes several TCP flags used in network communication: SYN is used to establish a connection during the three-way handshake between systems; ACK confirms receipt of data and identifies the next expected sequence number; PSH indicates the sender has data buffered and ready to be sent; URG assigns priority to urgent data; FIN closes a connection; and RST aborts a connection in response to an error. SYN scanning involves checking responses to SYN packets to identify open ports on a system using the SYN, ACK, and RST flags.