OK, so to clarify, both + and & can be used to perform string  concatenation.
& is preferred, because it specifically applies to strings, whereas + can be
used for both numbers and strings, which can lead to code ambiguity.

If you use &, you must put a space in front of the & when you type it. The
VBA editor will automatically reformat it so it has spaces on both sides.
You can type the + with no spaces around it, and the editor will put them
both in.

To show an example of the ambiguity, the following two functions produce
different results:

Sub s()
Dim a As String
Dim b as Integer
Dim c As Integer
a = "5"
b = "6"
c = a + b
Debug.Print c
End Sub

Sub s2()
Dim a As String
Dim b as String ' b is now a String also
Dim c As Integer
a = "5"
b = "6"
c = a + b
Debug.Print c
End Sub

whereas, using & always yields "56".