What is R-CHO in chemistry?

It is an indication of the ALDEHYDE functional group.

R-CHO

'R' means the 'rest of the molecule'.

When 'CHO' are arranged in such a manner in a formula , it means it is an Aldehyde.

The structural arrangement is R- C(=O)-H The 'R' The double bonded 'O' and the single bonded 'H; are all directly bonded to the carbon.

The is is the standard (IUPAC) international formula for aldehydes.

Similarly Alcohols are R-CH2OH

Simialrly Acids are R-COOH

Notice the arrangement of the elemental letters.