What is the meaning of 练 in 冬练三九？
Guifan


  冬练
  
  动 根据冬季气候特点进行体育锻炼, 以增强中枢神经系统的体温调节功能, 改善肌体抗寒能力, 增强体质。


Basically: winter exercises/training (based at improving thermal workings of the central nervous system, etc.)

Hanyu Da Cidian


  冬练三九，夏练三伏
  
  指不管严寒酷暑,坚持刻苦锻炼。三九,一年中最冷的时期。三伏,一年中最热的时期。


Sanjiu is the coldest part of the year - sanfu is the hottest part of the year. So: no matter how adverse weather conditions are one must persist in painstaking physical exercise.

Here's more examples from Hanyu Da Cidian:


  梁斌《烽烟图》
  这是练来的!我爸爸曾对我说过,叫我冬练三九,夏练三伏! 
  
  新凤霞《童年纪事》
  学戏练功吃苦可真不少,冬练三九,夏练三伏。 
  
  又《艺海博览》
  青年演员要做到拳不离手,曲不离口。冬练三九,夏练三伏。练出来没有用不上的。 


Just to clarify the times of year:

A Chinese-English Dictoonary


  三九
  
  the third nine-day period after the winter solstice-the coldest days of winter


&amp;&amp;


  三伏
  
  the three fu-the three hottest periods of the year (i.e. 初伏 chūfú, 中伏 zhōngfú, and 末伏 mòfú; altogether 30 or 40 days)


&amp;&amp;


  出伏
  
  the first day of the first fu (falling in mid-July)


&amp;&amp;


  中伏
  
  the first day of the middle or second fu (falling in late July)


&amp;&amp;


  末伏
  
  the first day of the last or third fu (falling in early or mid August)