Base strength of a species is its ability to accept H+ from another species (see, Brønsted-Lowry theory). The greater the ability of a species to accept a H+ from another species, the greater its base strength. Organic chemists customarily compare the strength of bases using the strengths of their conjugate acids, measured as pKa.
eg: Base strengths of F ¯ and HS ¯
conjugate acid of F ¯ = F ¯ + H+ = HF
conjugate acid of HS ¯ = HS ¯ + H+ = H2S
HF is a stronger acid than H2S. The stronger the acid, the weaker the conjugate base, and vice versa. Thus, HS ¯ is a stronger base than F ¯.