Hi,
Here are the common stress patterns. Please note that these differences are subtle rather than huge.
What IS it? What IS this? In both of these cases, the stress brings attention to the 'existence of the thing'. eg Perhaps the speaker wants to know its purpose. The verb is used in full (not abbreviated) in order to allow it to be stressed.
What is THIS? This stress brings attention to the 'thing' that is being talked about. eg Perhaps the speaker wants to know its name. The verb is used in full (not abbreviated) to 'separate' 'this' from 'what' and thus give it more stress. However, unless we want to use a great deal of stress, we very commonly abbreviate What is this? to What's this? Here, the speaker may or may give stress to THIS.
Note also that 'What is it? is not abbreviated to What's it?
Best wishes, Clive