I'm a university student studying statistics by 'Discovering statistics using ibm spss 4th edition'

1. I don't understand what 'content-validity' means even though i read the explanation from the book again and again...
2. There are two kurtosis types : positive(leptokurtic) and negative(platykurtic). But leptokurtic is heavy tailed, and platykurtic is thin tailed.

But leptokurtic is more thinner and pointy than platykurtic so I think leptokurtic is thin tailed... But it is heavy-tailed. So I found that I misunderstood the meaning of 'tail'

What does the tail mean?

I have two questions, thank you for reading, answering and saving me.