Learn Danish: Det er et stort skridt at sige op.

It gets a -t because it describes the neuter noun et skridt. In Danish, adjectives must agree with the gender of the noun they describe, so stor becomes stort.

How does sige op translate to 'quit'?

At sige op is a fixed phrasal verb. While at sige simply means 'to say', adding the particle op changes the meaning entirely to 'to resign' or 'to quit' a job, lease, or subscription.

How is the det in this sentence different from the det used in this lesson's abstract substantivation?

In this sentence, det acts as a dummy subject meaning 'it' (It is a big step). In the abstract substantivation you are learning right now (like det store ved...), det acts like the word 'the', turning the adjective into a noun phrase to mean 'the big thing about'.

Can I change the word order and put at sige op at the beginning of the sentence?

Yes, you can grammatically say At sige op er et stort skridt. However, just like in English, Danes usually prefer to put a placeholder det at the beginning and move the heavy action phrase (at sige op) to the very end of the sentence.