The hint says first birthday and this is a unit about days/calendar and yet it says the translation is rock?
I think something is amiss here
돌(乭) is rock for sure. Not sure if this helps, but the 돌 in this case, is from 돌다 (to spin around). Because time and seasons are circular, after a year you come back around to that day. Back in the days, it used to be written as 돐, which is how they still spell it up north.
Some words can mean multible things. They usually have a diffrent character in hanja.
Sadly it seems one has to deal with hanja if one wants to become an advanced level in the Korean language.
People really don't use hanja, so you can get really far by just differentiating them based on context. Unless you want to go into an academic study of Korean, you really shouldn't need to learn it.
However, if you're also considering learning Japanese or Chinese, it may be worthwhile since there's a lot of overlap.
돌 can mean either "first birthday" or "rock, stone", it's just two words of different origin that happen to look the same in modern language (like English "sea" and "see" happen to have the same sound). It was probably the "first birthday" meaning that caused it to be included in this lesson and then somebody rightly complained that the translation "rock" is also correct. I don't know why you got corrected though; "first birthday" was accepted for me.
For words as this, you just need to memorize rather than trying to figure out why. lol
돌 = stone or 1st birthday
for 1st birthday, people usually say 돌잔치(돌+잔치, 1st birthday + celebration/party) rather than 돌
Dont over think it. This app gives additional meanings and synonyms under each word. The meaning changes with context. That's all