It says don’t have any other gods before / besides God. Meaning, do not believe in any other gods. Don’t believe in them as intermediaries (“before”) and don’t believe in them as separate entities (“beside”). Meaning, don’t believe in them at all.
Technically, the verse says “don’t believe that other gods exist” rather than “other gods don’t exist”. But this is just one verse. Look at it in context. Throughout the Bible, it says that other gods are “nogods” and just figments of people’s imaginations.
jealous God: this is a notorious mistranslation. The Hebrew here implies strength and zeal of a paternal figure. The English “jealous” implies weakness and pettiness.