What does oxidizing strength mean?
The strengths of oxidizing and reducing agents are indicated by their standard electrode potentials. Let's consider  standard potentials of some redox couples.

$ E^0(\ce{Li^+/Li})= -3.04V$,

$ E^0(\ce{K^+/K})= -2.92V$,

$ E^0(\ce{Cu^2+/Cu})= +0.34V$,

$ E^0(\ce{F_2/F^-})= +2.87V$

The values above are reduction potentials, so lithium at the top of the list has the most negative number, indicating that it is the strongest reducing agent. The strongest oxidizing agent is fluorine with the largest positive number for standard electrode potential.