坚强 is an adjective meaning "strong," "firm," or "resilient," typically describing mental or emotional toughness rather than physical strength. It is used to encourage perseverance in difficult situations, as in 要坚强 (be strong). It can also modify nouns, such as 坚强的人 (a strong/resilient person).
你一定要坚强,别放弃。
Nǐ yīdìng yào jiānqiáng, bié fàngqì.
You must stay strong and don't give up.
她是一个非常坚强的人。
Tā shì yī ge fēicháng jiānqiáng de rén.
She is a very strong person.
经历了那么多困难,他变得更坚强了。
Jīnglì le nàme duō kùnnán, tā biàn de gèng jiānqiáng le.
Having gone through so many hardships, he became even stronger.