Similar to charring, toasting is a way of treating (burning) wood to prepare it for ageing spirits. The various levels of heat and burn have different effects on the compounds, and therefore, the type of flavours the oak imparts to the maturing spirit as well as the pace at which the interaction is happening. Toasting is considered to be the lightest form of heat treatment that is done to a cask.
It’s unusual to find American Oak (Quercus Alba) that has only be toasted due to the preferences and practices of the Bourbon industry. Toasting is quite a common level of preparation to apply to European oak (Quercus Robur) used for the likes of Cognac however, which only requires a light touch to enable the type of influence the distilleries require.