The questions everyone asks first
What actually is this letter? A formal notice under the Party Wall etc. Act 1996 — your neighbour plans work on or near something you share (a wall, a floor, the boundary). It protects you both. It doesn't mean anything is wrong, and it isn't planning permission.
What are my options? Three: consent in writing (most people do — you keep every right the Act gives you); dissent and appoint your own surveyor; or dissent and share one "agreed surveyor" with your neighbour, which is usually cheaper and faster.
What if I just ignore it? After 14 days, silence counts as dissent (section 5). A dispute formally exists and surveyors are appointed — doing nothing doesn't make it go away, it just takes the choice out of your hands.
Will this cost me money? Normally no — the building owner (your neighbour) pays both sides' reasonable surveyor fees. That's written into the Act. Curious about the numbers? See what a party wall agreement costs.