What I'm not sure about is that it is the off-hand equivalent of RuneScape Gold the spider wand. It lets you perform spells using their off-hand slot. What exactly does this mean and should I bother playing with it? And, I do not have any idea what "off-hand slot" is.

It's likely that you've observed, players are able to dual wield weapons in the present. Before (as in pre-Evolution of Combat) with the exception of 2 hands weapons, your only options were to utilize a single handed weapon with either a shield or an offensive shield slot like a Defender or a God book although they were shields theoretically. So now you have an extra choice - you can wield a weapon and a shield for extra defensive bonuses (and can use a couple shield only defensive/constitution abilities) or you can dual wield weapons.

Dual weaponry is more offensive-focused evidently, and is slightly better than using two-handed weapons because you get somewhat better abilities. This is why most people favor dual crossbows more than a traditional 2 handed crossbow bow, or an wand and the shield slot as an offensive weapon (usually orbs like that of the Spider Orb) rather than staves (except in certain circumstances where it is only necessary to risk one weapon). It is generally recommended to view the orb as a kind of off-hand crossbow or sword even if it doesn't appear like an actual weapon.

It's the equivalent of the Spider Wand because it has the same tier. In addition, it's the matching off-hand offensive weapon in the Spider Wand. The player can cast spells through the off-hand slot, since it's weapon just as the case if you had two swords for example you'd be able to deal harm with both. As you advance in level, you'll likely purchase and use Batwing Btw, which is even better and is incredibly cheap as well.

Warpriest is one of the top armours in F2p however it's only available to Cheap RS3 Gold players..., unless you took part in the tournament several months ago, which is unlikely. It maxes out at tier 2 in f2p (was reduced by Jagex) which means it's not really the best armor in all conditions.