It doesn't really mean anything : this is not a sentence which you would ever come across in the real world.
The active voice statement is this:
"I felt someone touching my shoulder".
In the active voice, this is a normal, natural sentence which makes perfect sense. 'I' is the subject, and the sentence is telling us what you felt. However, this is not a something which we would ever put in the passive voice: there is no logical reason why we would even try to make 'someone' the subject.
The sentence which you have is so awkward and unnatural that it is almost meaningless. Where did you see this?