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.)
Is "jeruk" both a singular and plural form?
The answer is yes. As everything is context dependant in Indonesian, not grammar dependant, as in English.
There's no real plural in Indonesian. There's no "s" adding like in English to turn a singular into a plural.
The Indonesian language uses the dobling of a word to indicate it's a plural. For instance apel means "apple", and apel-apel means "apples".
When it's obvious from the context that you have a plural, you don't need to "mention" it. So you don't need to turn apel into apel-apel, you can simply use "apel" and have it as a plural, as it's obvious.
The dobling in "apel-apel" is only used when you want to make it clear that it is not a singular. In every other cases, you will use only "apel". They don't like to repeat an information, if you know it's plural, you don't need to mention it clearly, with apel-apel, as you already know it...
So, I like apple is not something that exist in English, if you mean that you like every fruit that is an apple on earth, you would say "I like apples", but in Indonesian, you don't need the "s" equivalent, you don't need to mention that "apples" is plural. It's obvious. You like the "apple" category.
If you had "I like the/this apple", you talk about a specific one, and it's not all the apples on earth, but only one of them. You would have the specific-determiner in Indonesian: Saya suka apel itu (or Saya suka apelnya)
Other example that proves that the "dobling-plural" is only used when you need to kill an ambiguity: The translation of "I have 2 children":
Saya punya 2 anak.
I didn't use the non-ambiguous plural anak-anak, as I mentioned "2", so I already know it's a plural, I don't need to make it unambiguous by dobling "anak". Note that using "anak-anak" here is not wrong, but we have to take the habit to understand that "2 anak" means also a plural.
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