"le/la/les" are definite articles in French. So if you want to translate it, you must put that notion in your translation.
The plural form is often used to design a group of people, "les puissants" meaning the group of people who have the power. What kind of power exactly depends of the context. In the same way "les riches, les pauvres" refers to the rich people or poor people, "les sportifs" to people who practise any kind of sport, etc. In that case, the adjective takes the place and function of a noun and is used to qualify the people you're talking about.
The singular form is less common. It refers to a precise person, but you will need a context to identify precisely who it is. For example, in a company, if someone says "le puissant", it could refer to the CEO.