Base strength of a species is its ability to accept H+ from another species. The greater the ability of a species to accept a H+ from another species, the greater its base strength.
For acids: the stronger the acid, the smaller the pKa
For bases: the stronger the base, the larger the pKa
For example: In HCOOH the conjugate base is HCOO^- which is more stable as it accept H+
For More details see here - http://www.sarthaks.com/401/how-compare-the-acidic-and-basic-strength-different-acids-and-bases-explain-with-examples