There could be more conditions that this view is not showing, but given what’s on the screen:
0 = don’t NAT whenever 192.168.123.0/24 talks to something in 192.168.33.0/24
1 - 3 are all masquerade rules. Masquerade means to nat the source IP address by changing the original source IP into the IP address of the interface the packet is being egressed through.
(i.e. make it look like the packet came from the Mikrotik itself, not the original sender)
So:
1 means to masquerade everything going out ether1-gateway, except if the src IP is in the 192.168.33.0/24 network
2 means to masquerade everything whose destination is 192.168.30.100
3 means to masquerade anything from 192.168.89.0/24 regardless of where it’s going.
Generally speaking, source NAT is used to allow some network behind a router (“inside” networks) to reach a network on the other side of the NAT when the “outside” network contains no routes back to the “inside” network.