Short answer
HubSpot's KB: syncs customer, company, product, order and cart data. Company sync needs Shopify Plus and matches on normalized company name. Meta fields and product variants don't sync. Installing adds the HubSpot tracking code to your store.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Check name formatting before the first sync, since punctuation is stripped when matching.
3. Related
Shopify integration and product variants.