Inbound means incoming traffic coming to your EC2 instances. For that you have to add inbound rule. For web server generally we use port 80.
Outbound means outgoing traffic from your EC2 instances. To connect internet or any browser you have to add outbound rule.
For a detailed, You can even check out the details of Inbound and Outbound with the help of AWS Cloud Migration.
Hope this will clear your doubt.