Understand an official letter and what it wants you to do

I've had an official letter and I need to understand it before I do anything.

It's from: the IRS, US
What's worrying me: I did freelance work last year and I think I forgot a 1099. Am I in trouble?

The letter:

Notice CP2000. We are proposing changes to your 2024 Form 1040. The income and payment information we have on file from third parties does not match the information you reported on your tax return. This discrepancy may cause an increase or decrease in your tax. Proposed amount due: $1,146. If you agree with the changes, sign the response form and return it by October 27, 2026. If you do not agree, send a signed statement explaining why, along with any supporting documentation, by the same date. If we do not hear from you, we will send you a statutory notice of deficiency.

Explain it in plain English:
1. What this letter is, in two sentences. What kind of notice, and how serious it usually is.
2. What it says has happened, and what it's asking me to do.
3. Every date, amount and deadline, in a table, with what happens if I miss each one.
4. The choices it gives me, laid out side by side. Don't tell me which to pick.
5. Any term I probably don't know, defined in one line.

Then:
- What I should gather before responding.
- Whether this is the kind of letter people usually handle themselves, or one where it's worth talking to a professional (an accountant, a lawyer, a free advice service) first. Say which kind of professional, and what to ask them.

Explain only what the letter says. Where the answer depends on my own situation or on rules you're not sure of, say so plainly instead of guessing. If I've misread something in my worry, gently correct it.

What you get: The letter in plain words, every deadline in a table, your options side by side, and whether it needs a professional.