A pre-filter is an activated carbon sheet or similar that you stick at the top of your filter so that any big particles/residue get stuck on that first, before going through the way more expensive and fine filter cartridge. You will have to replace them much more often, but they’re like $20 for a 4 pack so the cost is WAY less than the $250 actual filter.
Something like this (you may have to cut to fit): https://www.amazon.com/Breathe-Naturally-Replacement-Honeywell-Purifier/dp/B06Y5HMKBP