1.
formidable person, tough competitor, veteran
A person with exceptional skill or strength in a particular area. Often used admiringly to describe someone who is impressively capable or experienced.
2.
warrior, soldier
A brave fighter or soldier. This is the original meaning, now mainly used in literary or historical contexts.
Usage
In modern Japanese, sense 1 is more common. The word carries a tone of admiration and respect. Often used to describe someone who is impressively skilled or tough, whether in sports, business, or other competitive fields.
Pronunciation
The same kanji 強者 can be read as きょうしゃ, which is a more formal, Sino-Japanese reading meaning "the strong" (as opposed to 弱者, "the weak"). The native Japanese reading つわもの carries more of a sense of individual prowess.