The + is an arithmetic operator that is used to add two numbers but it will concatenate them instead if one or both of the arguments is a string. The preferred concatenation operator is &. But, do look both up in help so that you will know when it is appropriate to use the +.
=[field1] & " " & [field2]
Just looking at the code
=[field1] + [field2]
I would assume that you were adding the two fields rather than concatenating them.
