Griddy is literally just allowing you access to the wholesale market.
In TX, the wholesale market is what generators get paid by retailers, who then charge consumers a price. For example, in a typical TX day in May, your wholesale price translates to like 3 cents/kWh. My retailer charges 9 cents/kWh at all times, and pays me 9 cents/kWh for any solar I sell back to the grid. Some retailers charge like 12.5 cents/kWh on weekdays, and weekends are free. Some places charge you a peak/off peak rate based on time of day.
But, for all of these things, even on that typical day in May, the wholesale price increases as businesses start and temperature rises. Now, much of our power supply has been built to ramp up with that demand (wind is at it's most productive the same time that demand is highest, for example, as is solar, and gas is pretty responsive - a GT24B should take like 30-45 minutes to startup and run, and a 7HA might take an hour (GTs are smaller, 7s are bigger - both combined cycle gas style). So the price rises somewhat, but usually not a crazy amount. And depending on that day's wind and sun, sometimes supply will outstrip demand and you'll keep the gas plants down (especially because you never take the nukes offline and you rarely bring down the coal plants).
HOWEVER, sometimes, an event will cause some supply to come offline AND at the same time demand will spike (this week has been perhaps the most extreme example I've ever seen of this phenomenon). When that happens, and you're exposed to the actual market, the price skyrockets. Usually, TXU energy gets screwed there and still charges you their 9-16 cents/kWh, but if you're buying wholesale, that's literally the risk you signed up for.
I know this is a little dumbed down for anyone with like an actual science degree in this, but I've spent a couple years with power generation and distribution clients across the country, and in ERCOT, so have a decent understanding of how it works.
@belljr @matuski adding you since I see you asked while I was typing this up.