In hebrew, each latter has a numerical value:
the first letter= 1
the tenth letter, "yod" = 10
the eight one, "heth"= 8
you see, combined together you get the sum 18
the hebrew word for life, or being alive, is "hay" - pronounced like "bye" or like "bay" in "bay leaves"
it's written first "heth"and then "yod". We mostly don't write the vowels in Hebrew, and ' of course, from right to left ( that's the direction of writing in Hebrew) .
it doesn't matter that it's the 8 first, the sum is what counts.
wearing a pendant with this symbol has nothing to do with the Bible, it's a tradition,for example it's like wishing for a long life to the person you give to.