Lookup more similar to vlookup/hlookup in Excel than to a SQL where clause. The lookup conditions are expressed in terms of a match value and a field to search for the matchvalue.
From the manual
lookup(fieldname, matchfieldname, matchfieldvalue [, tablename])
Returns the value of fieldname corresponding to the first occurrence of the value matchfieldvalue in the field
Fieldname, matchfieldname and tablename must be given as strings (e.g. quoted literals).
The search order is load order unless the table is the result of complex operations such as joins, in which
case the order is not well defined.
Both fieldname and matchfieldname must be fields in the same table, specified by tablename. If tablename
is omitted the current table is assumed.
If no match is found, null is returned.