Isaiah (26:4) is usually translated: "Trust in G-d forever, for in Y-ah, [who is] G-d, is the everlasting rock." However, the prefix translated here as "in" can also mean "through" or "by"; and the divine name Y-ah is spelled yud-hei, the word for "rock" (tzur) can also be taken as a verb meaning "to form", and the word for "everlasting" ("olamim") can be taken to mean "worlds". Thus, the verse can be mystically translated as: "Trust in G-d forever, for with the letters yud and hei G-d formed the worlds." Based on this (and other verses), the Sages say that G-d created This World with the letter hei and the World to Come with the letter yud. (Menachot 29b)
Rebi Yehudah bar Rebi Eloy elucidated: The Holy One, Blessed is He, created two worlds, one with a Heh and one with a Yud. I do not know if the World-to-Come was created with a Yud and This World with a Heh, or if This World was with a Yud and the World-to-Come was with a Heh. But then it says,"These are the generations of Heaven and Earth in the creating of them (behibar-am) . . ." (Bereishis 2:4); don't read "behibar'am" (i.e., in the creating of them) but "b'Heh bar'am" (i.e., with a Heh they were created). Thus we see that This World was created with a Heh and the World-to-Come was created with a Yud. (Menachos 29b)
Anyone who knows what the Yud and Hey stand for? or how to define their meaning in these explanations?