you have misconception of what is an index.
you create an index to speed up your search, as an example using dlookup.
you create two indexes, one for codpers and the other for codmateria field.
the syntax for dlookup() function:
dlookup([the field to return]", "[the table]", "[the filter/criteria]")
for example you want to return the Notes1 field whose record has codper=1234 and codmateria=456789:
dlookup("[notes1]", "theTableName", "codper=1234 and codmateria=456789")
if the criteria is found on at least a record, it will return the Notes1 for that record.
otherwise the return value of dlookup will be Null.