Linguistically, "uncountable" simply means "cannot be counted"; it's often used for things that would be impractical to count -- the number of stars in the sky, the number of grains of sand on a beach, the number of leaves in a forest. (The word "countless" is also used as a synonym.)
There's another meaning in linguistics -- a "count" noun (sometimes but not often called a "countable" noun) is a noun that you can count. You can, for example, count Sandwiches (we have five people coming to lunch so I'll make ten sandwiches), but you can't count milk. (Milk is an example of a "mass noun"; I can't have ten milks. I have to measure milk instead and pour five "quarts of" or ten "glasses of" milk.)
There's also a more technical meaning in mathematics -- a "countable" number is a number that is either finite, or at most as large as the number of natural numbers (0,1,2,3,4,....). Yes, there are infinitely many natural numbers --- but infinity comes in different sizes and some sizes are larger than the size of the infinity of natural numbers. So the number of natural numbers is "countably infinity," while the number of sets of natural numbers is uncountably infinite. But the math gets hard and a little strange at this point.
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Uncountable
Yes, "stimulus" can be both a countable and uncountable noun. For example, "The government provided several stimuli to boost the economy" (countable) and "Increased spending acted as a stimulus for economic growth" (uncountable).
No, "significance" is typically considered a non-countable or uncountable noun in English. It is used to describe the importance or meaning of something as a whole rather than individual instances.
No, the proper noun Lionel, a name, is a countable noun; the plural form is Lionels. example: There are two Lionels in my family, my father and my cousin.
Transport is both countable and uncountable as a noun.
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The gerund painting is a countable noun, as in "There are 12 paintings in this room."
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few is countable
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The noun 'steel' is an uncountable (mass) noun, a word for a substance.