Why ethanol is less acidic than phenol?

This requires a fair amount of background theoretical knowledge to explain.

Basically, as I'm sure you are aware, ethanol is CH3-CH2-OH. The acidity of a molecule depends on how easily it loses its most acidic proton, which in this case is the hydrogen atom in the alcohol group.

Phenol is C6H6-OH where C6H6 is a benzene ring. Once again, the most acidic proton is the hydrogen atom in the alcohol group.

A molecule is more acidic if the anion formed upon deprotonation (i.e: when it loses its proton, H+), can be stabilised. The benzene ring is much more electron withdrawing than the alkane chain in ethanol (due to delocalisation of the Pi system) which means the resulting negative charge in the anion of phenol can be stabilised much more easily than in ethanol, hence why it has a lower pKa and is more acidic than ethanol.

I hope this helps and if there is anything you don't understand or wish to be explained further, just ask.