I'm currently working as shadow resource(non billable) to a project what does this mean
This is a fairly common practice in organisations - particularly when projects are in danger of running over-budget.
For example, in my first job my role was paid out of company "overhead" (I was supposed to support our engineers who then client projects). Sometimes, I would end up working directly on a project that was running out of time or money because the work needed doing, but the client couldn't be charged more money (this kind of decision is political and financial) and my time wouldn't be recorded in the project - though, of course, I was paid my regular salary.
All things said and done - whether you are billing or not is immaterial to you for things like engineering and development where you're typically salaried and the company is just accounting for the cost of a project. For sales and legal professions, though, as well as consulting roles, whether you were billing or not does matter, since you're typically commissioned rather than salaried.
This is the general answer for you:
Billable resource: You are a resource where you are being billed for a client, say 40 hours per week and 8 hours for a day (generally) where client pays for you (genereally $25-30/hr in India)
Non billable (shadow resource) Exactly opposite to billable resource, where you're not billed directly to client and you are being paid by the company, generally non-billable resources are new to a project where you work as a backup to a billable resource (as name says as shadow resource).
Hope it helps you. This term is generally used in Indian companies, I hope you're from India so asking this question :)