Learn Italian: Ho letto della polemica e ne scriverò. - I have read about the controversy and I will write about it.

It replaces the phrase della polemica. When a verb takes the preposition di to mean 'about' (like scrivere di, to write about), you can replace the whole di + noun phrase with ne. This prevents you from having to repeat 'about the controversy' in the second half of the sentence.

Why does the sentence use della instead of sulla for 'about the'?

While su is often used to mean 'about' or 'on the topic of', Italians very commonly use di (here combined with la to make della) with verbs of communication like leggere, scrivere, and parlare. Both are correct, but using di allows us to use the handy ne pronoun later!

Why is ne placed before scriverò?

ne acts just like direct and indirect object pronouns (like mi, lo, or gli). These small pronouns always go immediately before the conjugated verb, except in specific cases like commands or attaching to an infinitive verb.

Why are there two different verb tenses here?