A phenol is a hydroxyl (OH) group coming off an aromatic ring, similar in some respects to alcohols, but much more acidic. TLC-thin layer chromatography, where substances are separated out, and characterised by virtue of different retention times and speeds of movement relative to other substances in a mixture in given solvents/solvent mixtures, useful for, for instance, following a given reaction to see how its progressing, or if something is present at all.

Acetyl group is C=CH3, propionyl would be one more carbon: CH3CH2C=O, the functional groups relating to acetic acid (vinegar) and propionic acid respectively, the process for making H involves reaction of morphine base with acetic anhydride (usually) although acetyl chloride can be used, to form an (di)acetyl ester,  (esters are substances derived from an alcoholic or phenolic hydroxyl group  with an oxygen-containing acid via condensation,  having the formula RC=OOR for a carboxylic acid ester such as a formyl, acetyl or propionyl ester)

That increases the lipophilic character of morphine making it cross the blood-brain barrier much faster, and better, which is why heroin is known to have a greater rush when injected as well as greater potency as a painkiller in comparison to morphine (it acts as a prodrug to a considerable extent, 6-monoacetylmorphine is also pretty active and is produced in the body by metabolism from H)

AFAIK dipropionylmorphine is more potent still, and longer lasting, although, apparently according to wikipedia, slower acting, surprising, unless its due to the larger acyl group the ester is formed from proving slower to hydrolyse.