Lifecycle Stage is a default HubSpot property, designed to designate where contacts are in your marketing funnel. Each stage represents a transition down the funnel.
The stages, in order (from top of the funnel first to bottom of the funnel last), are: Subscriber, Lead, Marketing Qualified Lead, Sales Qualified Lead, Opportunity, Customer, Evangelist, and Other. For more detailed information on and definitions of each stage, please see this article.
Lifecycle Stage is a great benchmark to use in order to properly segment contacts within HubSpot. So, for example, you might choose to create email lists for each of your segments in order to send them stage-appropriate nurturing emails.