Single-rate means that the price you'll be charged per kWh of usage will not change by the time of day or by the day of the week that you use that energy.

The simplest tariff is the flat single-rate tariff where you'll be charged the same rate per kWh of usage no matter when you consumed the energy, and no matter how much you consumed over the billing period.

So-called 'single-rate' tariffs may not actually be single rate.

Some single rate tariffs are 'stepped rate', which means you'll pay a different price per kWh once you've crossed a certain threshold of total consumption for the billing period.