Service entrance wire is the wire that leads from the weather head to your electric meter.
The service entrance wire also leads from your electric meter to your breaker panel indoors.
Then from your breaker panel the regular copper Romex wire distributes the electricity through your house or building.
Service entrance wire can be either copper wire or aluminum wire.
The copper service entrance wire is a smaller wire size than the aluminum service entrance wire is.
Aluminum service entrance wire is usually a 2-0 wire for 100 amp electric service.
For 100 amp electric service for copper wire the service entrance wire is a # 4 AWG size wire.
Copper is a much better conductor of electricity and it's less prone to corrode than aluminum wire is.