The collector ID is the merchant's identity in Google Smart Tap. When an Android phone reaches a reader, the reader announces its collector ID, and Google Wallet looks for a pass whose class lists that ID among the merchants allowed to redeem it. A match means the phone sends the pass's Smart Tap payload; no match, nothing is sent.
Unlike the Apple VAS merchant ID, which is computed from the pass type identifier, a collector ID is assigned by Google. The merchant, or the wallet platform on the merchant's behalf, registers for Smart Tap and receives a numeric ID along with the ability to upload a public key. The private half of that key lives in the reader or payment terminal and is what decrypts the payload; the collector ID says who is asking, the key proves it.
The link to the pass is made on the class side. A pass class with Smart Tap enabled lists the collector IDs that may redeem its passes, and every pass object under that class inherits the list. This is what lets one program work across a merchant's estate and, where a program spans several merchants, lets each add their own ID without touching members' passes. See Smart Tap redemption for what the payload contains.
For a merchant setting up NFC identification, the collector ID is one of the two values a terminal vendor will ask for, the other being the Apple VAS merchant ID. Both are stable, both are safe to share, and both point at the keys that must not be. Whether Smart Tap is registered under the merchant's own Google account or the platform's is worth asking early, because it decides who can add or change readers later.