"This is an important letter."
Translation:C'est une lettre importante.
It is in the Duolingo "Notes" when you use the web site . . .
B is for beauty. Une belle femme — A beautiful woman
A is for age. Une jeune fille — A young girl
N is for number. Deux hommes — Two men
This can also be for rank: Le premier mot — The first word
G is for good or bad. Un bon garçon — A good boy
S is for size. Un gros chat — A fat cat
BANGS adjectives (Beauty, Age, Number, Good/bad, Size) is a memory aid to which words go in front but it is not every word that is good or bad related, indeed it is only "Number" related words (including first, second etc) that always goes in front, the full list is below:
beautiful: beau / belle
pretty: joli / jolie
old: vieux / vielle
young: jeune / jeunesse
new: nouveau / nouvelle
Any and all numbering goes first: un, deux, premier / première, deuxième etc (one, two, first, second etc)
good: bon / bonne
bad: mal / mauvais
Big: grand / grande
Small: petit / petite
As @pjbf1979 linked above it is actually trickier than that (Un grand homme grand is "a great tall man"), but the above will serve well for now.
C'est is not only for masculine nouns. It often means 'it is' or 'this is'. Examples: C'est une jolie pomme: This is a pretty apple. C'est Hélène: It is Helen.
Cette is an adjective that means 'this' or 'that', but is used only to refer to feminine nouns (ce or cet is for masculine nouns). Examples: Cette jupe est verte: This skirt is green. Elle adore cette cuillère: She loves this spoon.