Values in Flash can be numbers or strings. These values can be literal values or represented by expressions. Actionscript is not a "typed" language: a variable's type (string, integer, and so on) depends on the type of values assigned to the variable. Variable are also flexible and can be treated as either a string or a number depending on the type of operator used on it.

In  "x" = "3" + "4"  --  x would be  7
In  "x" = "3" & "4"  --  x would be 34 

+  is a numeric operator so you just do standard addition.
&  is a string operator and you treat the 3 and 4 as characters and not numbers and the characters are concatenated (chained together).

With Flash 4 you will be able to enter new values below:

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In Flash if you select String Literal (abc), Flash will automatically add quotes to the string literal or variable.

If you select Expression (=) you can have a number (22), a string literal ("John"), a variable that represent a number or string (string), a numeric expressions (x+1) or a string expression ("John " & "Croteau"). In expressions (=) you must add any needed quotes to strings when needed.