Rhyming doubles for things in general: cây cối, máy móc
… and the like
Pattern: [noun] + [rhyming copy] (cây cối, máy móc)
Some nouns come with an echoing second half that has no meaning of its own: cây cối (trees and plants), máy móc (machinery), bạn bè (friends), đường sá (roads), chim chóc (birds). The pair means the whole category, things of that kind in general, a bit like English "greenery" next to "plant".
The second half only exists inside the pair: cối in cây cối and móc in máy móc add nothing you could translate. That's what sets them apart from pair words like quần áo (clothes, from trousers + tops), where both halves are real words.
Because they mean a whole category, you can't count them. Don't put a number or a classifier in front: not hai cây cối but hai cái cây or hai chậu cây (two pots of plants). To talk about one, use the plain noun.
They're ordinary everyday words, and bạn bè is one of the most common words in the language. A few carry a colour of their own: giấy má (paperwork) usually comes with a sigh.
Don't confuse them with a doubled adjective like đo đỏ (reddish), which softens a quality rather than gathering things into a group.
Examples
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Nhà ông bà ngoại ở quê có nhiều cây cối lắm.
My grandparents' place in the countryside has lots of trees and plants.
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Bạn bè em ai cũng lập gia đình hết rồi.
All my friends have got married now.
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Máy móc trong xưởng này cũ quá rồi.
The machinery in this workshop is really old now.
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Đường sá ở đây mới làm lại, chạy xe êm lắm.
The roads here have just been redone, it's a really smooth ride.
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Giấy má rắc rối quá, em làm cả tuần chưa xong.
The paperwork is such a hassle, I've been at it all week and it's still not done.