HubSpot contacts with a Lifecycle Stage of Subscriber will not sync with Salesforce by default. This allows you to maintain a segment of contacts who need more nurturing in HubSpot before they sync to Salesforce. For example, you may import contacts from a trade show into HubSpot, but you don't want them to sync to Salesforce until they engage with you by filling out a form on your site.
However, if an inclusion list is present, Subscribers will sync to Salesforce as long as they meet the criteria of the inclusion list. For instructions on setting up an inclusion list, check out this article.