What is the meaning of 磨练? When researching online, finding an exact definition was hard. 

Does it mean to make oneself hard? Or like to harden yourself?

Could you give me a few examples?
磨练： temper oneself hard


在身为罗马奴隶的艰苦磨练中，宾汉变得更强壮、更有耐力。

Ben-Hur's years of enduring hardship as a Roman slave strengthened him and increased his endurance.

唯有艰苦磨练才能强化心灵

only through experiences of trial and suffering can the soul be strengthened

Sometimes, you might see 磨炼(smelt) instead of 磨练(exercise/training). There are some disputation on which one is correct. But main idea is that 磨炼 is sort of wrong words, but you may understand it as a synonym.