All your contacts have a lifecycle stage applied to them (or they at least should).
Knowing where a specific contact is in your processes, allows you to understand better how to handle them, and know which part of the process they are.
Imagine a potential lead, who has actively engaged with you and could be in a position to buy, but is not categorised, they could fall by the way side.
The HubSpot default Lifecycle Stage property are listed below:
The lifecycle stage of a contact or company can be changed by automatic syncing and therefore cannot be edited. Learn how to create a custom lifecycle stage property.
To keep the Lifecycle stage property updated, you can do the following:
Please note: a contact's lifecycle stage is only meant to progress forward, down the funnel. Imports, workflows, and form fields can only set the default Lifecycle Stage property to a lifecycle stage down the funnel. You can only set a lifecycle stage backwards manually, or in a workflow by clearing the Lifecycle stage property prior to setting the new value.
More on Lifecycle Stages can be found - https://knowledge.hubspot.com/articles/kcs_article/contacts/use-lifecycle-stages