The blackbody radiation is radiation
with not only one frequency (e.g. like in a laser beam).
Instead it is a mixture of many frequencies
(essentially all frequencies from $0$ to $\infty$).
Therefore it doesn't make much sense to ask for the radiation
power at a single frequency $f$.
But it makes perfect sense to ask for the radiation power
in a frequency range (from $f_1$ to $f_2$).
Power per unit area and per unit frequency just means
the power emitted from an area of $1$ m$^2$
in a frequency range of $1$ Hz
(for example in the range from $500$ THz to $500.000000000001$ THz).
The spectrum below shows the radiation power per
area (measured in m$^2$) and per frequency range (measured in Hz)
as it varies depending on the frequency (here given in THz).
image from Energy eduation | Blackbody radiation