Next action fields help track the next pending activity on a Lead, Contact, Company, Opportunity, Project or Helpdesk ticket.

In Settings > Additional Settings > Next Action Settings you can control which activity types update the next action field by ticking/ unticking the relevant box.

The next Pending Activity will then display in the System Information block on the relevant record:

You can also see the Activity in the Pending sub-tab, where you are able to click on the Activity or magnifying glass to view the Activity in detail.

Contact next Action on Linked Company

You are also able to have a Contact's "Next Action" field populate the linked Company's "Next Action" field by ticking the box: "Update Next Action field on contacts linked company even if company not linked directly to activity"