What does N' stands for in a SQL script ? (the one used before characters in insert script)

N is used to specify a unicode string.

Here's a good discussion: Why do some SQL strings have an 'N' prefix?

In your example N prefix is not required because ASCII characters (with value less than 128) map directly to unicode. However, if you wanted to insert a name that was not ASCII then the N prefix would be required.

INSERT [dbo].[TableName] ([Sno], [Name], [EmployeeId], [ProjectId], [Experience]) 
VALUES (1, N'Wāhi', 'ESD157', 'FD080', 7)

The "N" prefix stands for National Language in the SQL-92 standard, and is used for representing unicode characters.

Any time you pass Unicode data to SQL Server you must prefix the Unicode string with N.

It is used when the type is from NVARCHAR, NCHAR or NTEXT.

For more info refer to this: Why do some SQL strings have an 'N' prefix?

'abcd' is a literal for a [var]char string (or maybe text, but varchar(max) would be more common now) - occupying 4 bytes memory, and using whatever code-page the SQL server is configured for. N'abcd' is a literal for a n[var]char string (or maybe ntext, but nvarchar(max) would be preferable), occupying 8 bytes of memory using UTF-16. This allows for full international usage, and frankly n[var]char should probably be the default in most systems.