I'll try to make it straightforward, without being patronising, as I understand it.
It's all about the difference between (a) the total of the VAT paid on goods & services you bought (everything purchased - goods for re-sale, office supplies, other materials, utilities etc.) and (b) the total VAT you charge your customers for the stuff you sell.
If (a) is less than (b) then you pay HMRC the difference, if (b) is less than (a) then they give you a "refund" of the difference (or just carry it forward to the next VAT quarter).
Basically you are acting as a tax collector for HMRC and collecting the VAT on their behalf for the stuff you sell to your customers. You get to take off any VAT you paid out on stuff you bought, from this figure.
Make sense to you?