ASA(config)#domain-name cisco

with this command we define domain-name to be used when generating crypto keys.

ASA(config)#crypto key generate rsa label cisco modulus 1024

with this command we create crypto keys on asa, naming it "cisco" and also defining key size with modulus "1024".

ASA(config)#ssh 0 0 inside

with this command we define from inside of ASA traffic of ssh will be initiated and with 0 0 we define any ip address and any subnet mask. we can also specify particular ip address and also ip network along with interface name, from where traffic will be initiated. 

ASA(config)#username ABCD password ABCD1234 privilege 15

with this command we define username and password for SSH user and also privilege level.

ASA(config)#aaa authentication ssh console LOCAL

with this command we define authentication method to be used when any user try to login for SSH and "LOCAL"  key word defines local database to be checked for username and password (LOCAL in capital keys as it is case sensitive), we can also use AAA server using RADIUS or TACACS+ servers for database of user