"re-entry stamp in passport valid minimum 3 months after expiry of the visa"
what does re-entry stamp in this sentence mean?
This provision probably does not apply to you, since you are a Somali citizen (as you have indicated in the comments).
The provision, I just now realized, seems to be designed to ensure two things: First, that the applicant is legally resident in Kenya, and second, that the applicant will be able to return to Kenya after the expiration of the Schengen visa. (That is, I suspect that the "visa" referred to in "minimum 3 months after the expiry of the visa" is the visa for which you are applying.)
For those resident in Somalia, the facts of residence and admissibility presumably need to be proved with respect to Somalia rather than Kenya. As a Somali citizen, you obviously require no residence permit or visa or other passport stamp before being admitted to Somalia, so you should probably just ignore this item on the list.
Of course, a zealous and ill-informed employee of the consulate or of VFS Global might not understand the reasoning I've just outlined (if it is indeed correct). I therefore suggest that you ask someone whether that list item applies to you, or if you should substitute something else. If they say that you need to provide a residence permit or show a stamp, explain your situation and why those things do not apply to you.