Hi sabahatu1546489 and Frank Breen!
It's helpful to think about this in terms of 'recommendation delivery' and 'entity ingestion':
Delivery:
Many types of recommendation criteria require a "key" off which to base the items to be recommended. For example in a "people who viewed this viewed these" criterion, 'this' is the key. Your page needs to tell us, in the mbox request, the key. In this example, the key is the item being viewed, specifically it is the unique identifier for that item, the entity.id. Once Target knows what the visitor is viewing, we can return the items to recommend.
Please note that not all criteria are key-based. For example "Recently Viewed" and "Top Sellers" do not require a key, and thus will work fine without passing entity.id.
Ingestion:
Entities (we use this term instead of 'products' because we can recommend other things like articles, videos, etc.) can be ingested into Target via feeds, mbox calls, api calls, or any combination of these. The "entity.id" is an identifier that uniquely identifies the entity, so in your case sabahatu1546489 it is likely your productID. Mbox calls are primarily used for delivery, however a lot of customers find it convenient to piggyback off mboxes and pass in the full set of entity attributes on their mbox calls. However, if you pass in your full product catalog via feeds, you only need to pass a key in your mbox request and then only if you are using a key-based criterion.
I hope this helps!
Chris