Security roles, Permissions

Execute permission serves to give permission to execute the object.
"Tabledata" is NOT an object but it is the data in the object "Table". So you execute code of the table but not the data in the table.
In short : Table,Form,Report,Dataport,Codeunit,Menusuite,XMLPort : you need to give users execute permission, because these are objects. They don't need to read/insert/modify/delete the objects.

Users need also to read/insert/modify/delete the records in the tables (this is tabledata).

Empty=no permission
Yes=permission
Indirect=(for tabledata only) you have permission if some objects has in its properties the permision to RIMD data in the table. E.g. User can post invoices. The posting codeunit has also permission to write in table 17 G/L entry. But the user cannot directly change the table. To be able to post something, the user needs indirect permission in table 17.