Baseload in the context of power means the minimum level of power needed for a power grid. To supply baseload power, generating plants need to be able to generate a constant level of output. This is doable for a nuclear, coal or gas-fired plant, but not possible with most forms of renewable energy generation. Suitable power plants may contract with the grid to supply baseload power. Often, this is done on a day-ahead basis – where generators and off-takers submit bids to an exchange such as Nord Pool.
Sales of non-baseload power in the spot market in the UK are made at the energy imbalance price. There is a system buy price for participant in need of energy and a system sell price for participants with excess power to sell. In times where power is in short supply, such as when the weather is cloudy but there are low wind speeds, these prices may be much higher than when there is too much power. Prices can be negative at times.
If you want a more technical explanation – this guide from Elexon may help.