What exactly did God mean by saying “other Gods” ?

Exodus 20:3-6 3 Thou shalt have no other gods before me. 4 Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth: 5 Thou shalt not bow down thyself to them, nor serve them

As God knew none exists, then…