Each Airtable record has a unique ID, so searching by an ID will always return a single record, never multiple records. If you have three IDs, those relate to three records.
As explained in the text below that field, it needs to be a valid Airtable formula, the same type of formula you would create directly in a formula field. The advantage here, though, is that you can insert data from previous steps in the scenario. For example, if a previous step outputs a record ID, you could use something like this:
RECORD_ID() = "insert record ID data from previous step here"
For any record where its ID matches the ID from your previous step, it will be collected and passed on to the next step.