what does it mean when declare a string with more than one value?

Question

here is an example,

public string a_UId, a_ProgramID, a_BuildingID, a_UserName, a_Level;    
public long b_Linetype= 0, b_TotalLine = 0;

Answer 1

You're not "declaring a string with more than one value". You're declaring several different variables. It's really important to differentiate between variables and values.

So:

public string x, y, z;

is equivalent to:

public string x;
public string y;
public string z;

(As an aside, I'd strongly advise you to ditch the prefixes, and make all fields private.)

Answer 2

it is a shorthand for:

type variable1;
type variable2;
...