Chemguide: Support for CIE A level Chemistry
Learning outcome 17: Carbonyl compounds
17.1: Aldehydes and ketones
This statement is about testing for the carbonyl group in aldehydes or ketones using 2,4-dinitrophenylhydrazine.
2,4-dinitrophenylhydrazine is often abbreviated to 2,4-DNP or 2,4-DNPH. CIE use the 2,4-DNPH version.
When an aldehyde or ketone is tested with a solution of 2,4-DNPH, you get a yellow or orange precipitate. This is used as firm evidence of the presence of the carbon-oxygen double bond in these compounds.
In other words, if you add 2,4-DNPH to an unknown compound, and get a yellow or orange precipitate, then the compound is an aldehyde or ketone.
CIE's teacher support material implies that this is all you actually need to know, and so far there haven't been any questions which expect any more than this.
However, it seems to me to be wrong to introduce a reaction as if it was some sort of magic recipe, and I would suggest that you read the Chemguide page about this reaction, without taking any trouble to learn it, just for your own satisfaction. Ignore the bit at the bottom of the page about "Some other similar reactions".
© Jim Clark 2020