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
&&
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)
&&
出伏
the first day of the first fu (falling in mid-July)
&&
中伏
the first day of the middle or second fu (falling in late July)
&&
末伏
the first day of the last or third fu (falling in early or mid August)