A SIM farm is a bank of consumer SIM cards used to send application traffic as if it were person-to-person messaging; operators detect and block it, and its senders lose numbers and names.
SIM farms exploit consumer plans with unlimited texting to push bulk traffic at retail prices. Operators see the fingerprints (volume, identical content, no replies, rotating SIMs) and block the SIMs, the content patterns and sometimes the brand names involved. Recipients see random mobile numbers as senders.
They are the classic grey route and the reason "anonymous" bulk SMS has a bad reputation: the anonymity is on the sender side of a channel that was never meant for A2P.
At SMSMeteor: no consumer SIMs are used; traffic runs on A2P routes with the sender rule of each destination. The anonymity guide explains what carriers and recipients still see.