In this case it refers to oranges as a whole. 'I like oranges (in general)' as opposed to 'I like this orange' (saya suka jeruk ini). For plurals the noun is doubled, though this usage is just for times when the plural is not obvious from context or in really formal speech.
Complex example from a news story: "Pengunjung menimbang jeruk-jeruk yang baru mereka petik dan akan dibawa pulang." (Visitors weigh the oranges they have just picked and will take home.)
So how should I've known to use the plural form of orange instead of the singular form, based on the word "I" and "like"?
here is a simple example:
1) I like oranges (saya suka jeruk) 2) I like this orange (saya suka jeruk ini) 3) I like two oranges that you give me (saya suka dua (buah) jeruk yang kamu beri) 4) I like oranges from grandma garden (saya suka jeruk-jeruk dari kebun nenek)
Indonesian is a simple yet confusing language because it's heavily based on the context. for the plural form, indonesian used reduplication form and counter form. the reduplication form you can see it in 4, meanwhile the counter form you can see it in 3. hope this help you
Because the context in this sentence is supposedly about oranges in general, not specifically determined to a specific orange (e.g. this/that orange). As a Bahasa Indonesia native speaker though, I can understand; I almost fell to that "singular trap" myself
konteksnya itu adalah jeruk secara keseluruhan makanya memakai s. kalau konteksnya menyukai satu/spesifik jeruk, baru tidak memakai s. begitu kak