Learn Russian: Октябрь и ноябрь — осень.

In Russian, the present tense of the verb 'to be' (am, is, are) is almost always left out. When a sentence connects a noun subject to a noun definition (like stating X is Y), Russian writers often use a long dash to replace the missing verb.

Why are ноябрь and осень written with lowercase letters?

Unlike English, Russian does not capitalize the names of months, days of the week, or seasons. The only reason Октябрь is capitalized here is because it is the very first word of the sentence.

Октябрь, ноябрь, and осень all end in a soft sign (ь). How do I know their genders?

Nouns ending in a soft sign can be either masculine or feminine, which usually requires memorization. However, a great shortcut for beginners is that all Russian months ending in a soft sign, like октябрь and ноябрь, are masculine. The season осень is feminine.

Since the subject is plural (October and November), why isn't осень plural?

You are grouping the two months together and placing them into a single category. Just as you say 'autumn' in English instead of 'autumns', Russian uses the singular form осень to name the season they belong to.