SGT means Security Group Tag (original definition) or Scalable Group Tag (updated term which has not been universally used even within Cisco).
An SGT is appended to an Ethernet frame as an identified to allow switches and routers to apply policy (such as an ACL) based on the tag rather than based on the traditional 5-tuple (protocol, source IP, source port, destination IP and destination port).
SGTs are technically appealing since they use much fewer hardware resources in the switches and routers. Generally the resource in short supply is TCAM (Tertiary Content Addressable Memory). When we adopt segmentation based on per-port/end device access control, we can quickly run out of available TCAM.