Sorry if my question is too basic and possibly trivial. I'm new in Stackexchange sites.
My question simply is:
- Can someone ask a question in Meta or answer a post without registration?
No, this is not possible. Participation on meta is a privilege one gets with 5 points on this site, so that a completely new account cannot post on meta.
A full list of point requirements is available.
Note that the point requirements are not the same over all sites. This site is in "beta" so the requirements are significantly lower than on "graduated" sites (and even among those there is minor variation and some exceptions). For the specific privilege you ask about, participating on meta, the point requirement should be the same, or almost the same, over all sites.
A notable exception is Meta Stack Exchange, the general meta-site, which technically is a site in its own right, and where one thus can sign-up and post without prior participation on another (main) site. Posting for unregistered users is however not possible there.