I was looking at my dictionary to look up the word "writing" and it said it is an uncountable noun.

OK, but it has a plural form "writings" right beside the word "writing." I presume the plural word "writings' is an uncountable too like its singular form "writing." Can you help me to understand by giving me some examples of using "writings" as an uncountable noun?
Writings = n(plural) the books, stories, poems etc. that a particular person writes: Mark Twain's writings.
Thank you. How do we know if a singular noun or a singular collective noun is countable or non-countable in nature?

According to my dictionary, a singular noun is always singular and must have a word such as "a" or "the" in front of it.

Examples given by the Collins/Cobuild dictionary:

The sun was shining.

His grey socks were in the wash.