The Price Cap sets a limit on the maximum amount suppliers can charge for each unit of gas and electricity you use, and sets a maximum daily standing charge (what you pay to have your home connected to the grid).
That means there's no upper limit to what you actually pay – if you use more energy, you'll pay more, use less and you'll pay less.
It only applies to providers' standard and default tariffs, so if you're on a fixed-term energy deal, the Cap doesn't apply (although currently those who prepay for their energy do still get a small discount on rates under the Energy Price Guarantee). If you've not switched in the last year or so, it's likely you're on a capped tariff.