It means 'choose the appropriate thing for the situation'. For example "When deciding what is the best transport available it's horses for courses really. If you have a big family you might want to rent a large car, but if you are on your own then you might hire a bike." But people often use it to justify advice/their opinion. For example "If you're on holiday in London you shouldn't rent a car or bike, use the underground. It's horses for courses."

Go to this link https://fraze.it/n_search.jsp?hardm=1&t=0&l=0&p=4&q=horses+for+courses and then find the original articles that contain the phrase. After seeing the context for this phrase in several of them, you should be able to work out the meaning and context well. In terms of form, we either say the phrase on its own or add "it's" at the front. The phrase describes the situation and does not need any more grammar to make sense. Here's my example: "If you want to study urban American slang, then work with a younger American teacher of English. But if you want to speak like Benedict Cumberbatch, then you might want to find a British pronunciation coach. It's horses for courses."