fbsql_result() returns the contents of one cell from a FrontBase result set. The field argument can be the field's offset, or the field's name, or the field's table dot field's name (tabledname.fieldname). If the column name has been aliased ('select foo as bar from...'), use the alias instead of the column name.
When working on large result sets, you should consider using one of the functions that fetch an entire row (specified below). As these functions return the contents of multiple cells in one function call, they're MUCH quicker than fbsql_result(). Also, note that specifying a numeric offset for the field argument is much quicker than specifying a fieldname or tablename.fieldname argument.
Calls to fbsql_result() should not be mixed with calls to other functions that deal with the result set.