You will likely schedule a time to sit down and discuss the letter and they might give you some indications of how to proceed.
At that point if you want to continue forward a "retainer" fee will be collected. It could be thousands.
From there good luck!
Get ready for so many continuances and discovery requests. Do not delete anything, purposeful deletions can be taken as an admission of guilt.
But I’m clearly not your lawyer or a lawyer just someone who has been sued.
They always want to settle for precedent. Prior art is a harder thing than it should be, but you better find out everyone else they sued and make contact, assuming they aren’t gagged.
Sadly courts don’t slammed down enough people as vexatious litigants, so here we are.