Comma-separated join (cross join)
The comma is the implicit join operator, ANSI SQL-89 syntax. No different than the comma in: 

select ... 
from a , b             -- &lt;-- this comma
where a.aid = b.aid; 


I think you can replace it with CROSS JOIN*, assuming a recent enough Oracle version.

The i, it and definition_xml are the aliases of the 3 joined tables (the 2 tables are base tables or views and the 3rd is something like a derived table, Oracle calls them "table collection expressions") but they are no different regarding how they can be joined or referenced.

The parentheses around the joins and the last table are completely optional. I added them just to show how the query is parsed:

SELECT       
    EXTRACTVALUE(definition_xml.COLUMN_VALUE, ...) AS domain_name 
FROM        
      (   SDE.GDB_ITEMS_VW i 
        JOIN 
          SDE.GDB_ITEMTYPES it 
        ON
          i.Type = it.UUID
      )
    ,                           -- or: CROSS JOIN 
      (  TABLE(...) definition_xml
      ) 
;




* CROSS JOIN and the comma join operator have different precedence but this doesn't matter for the specific query. As a good practice though, it's best not to mix the comma join with explicit JOIN.