Lifecycle Stage is the position of a contact in the prospect-to-customer journey: Subscriber -> Lead -> MQL (marketing-qualified lead) -> SQL (sales-qualified lead) -> Opportunity -> Customer -> Evangelist. The exact stage list varies by company.
The discipline that makes Lifecycle Stages useful: a documented definition for each stage, a documented entry + exit criterion per stage, a governance owner for the property values, and an SLA per stage transition (Lead-to-MQL within 7 days; MQL-to-SQL within 14 days; SQL-to-Opportunity within 21 days). Stages without definitions become buckets that everyone interprets differently.
In HubSpot, Lifecycle Stage is a default property that ships with the system; in Salesforce + custom CRMs the equivalent is a status field. The platform doesn't define the stages -- the GTM team does.