What is business cycle frequency?
Business cycle frequency is a frequency that has the length of one business cycle.  Business cycle is:

economy-wide fluctuations in production, trade, and general economic activity. From a conceptual perspective, the business cycle is the upward and downward movements of levels of GDP (gross domestic product) and refers to the period of expansions and contractions in the level of economic activities (business fluctuations) around a long-term growth trend.

The length of the business cycle varies but usually, it is at least a few years.