Well in general when you are referring to the Supreme Lord/Devi you use the word "Ya" as in the verse stated above and others out there in Sri Rudram for instance:
या तेरुद्र शिवा तनू-रघोराऽपापकाशिनी ।
तया नस्तनुवा शन्तमया गिरिशंताभिचाकशीहि ॥ १-३
Ya te Rudra Shiva tanura ghora papakashini
taya nastanuva shantamaya girishanta bhichakashihi
O Lord Rudra, One with terrible form, destroyer of sins
With your peaceful, auspeciousness bless us.
Or this one:
यामिषुं गिरिशंत हस्ते बिभर्ष्यस्तवे।
शिवां गिरित्र तां कुरु मा हिसीः पुरुषं जगत् ॥ १-४
Yamishum giri shanta haste bibharsya stave
shivam giritra tam Kuru ma higmsih purusham jagat
O Lord living in the Mountains, giver of good things.
Make us peaceful, let the material world exist
Also as in Devi Suktam:
या देवी सर्वभुतेषु Ya Devi Sarvabhutesu
means O Devi of all souls, creatures
In other words it's part of the salutation. The consonant य् (y) with अ (a) added to it merely for the sake of facility in pronunciation