I placed a new weapon in my primary weapon slot and the background turned yellow...

What does this mean?

Am I still able to use the weapon? Should I use it?

It means you get no stats from the weapon because you are lower than the required level. I don't know how it works for weapons, whether you won't even swing at all or whether it will read it as a 1 dmg weapon or something, unsure on that.

I tried it...it wont even swing. Well...how do I know when I can use it? It made the background yellow...so...couple levels? or what?

Look for the Required level of ## on the item display. That is the minimum level you have to be.

Yther Ore.