Pending simply means that your e-filed return is on its way and that the government hasn't accepted or rejected it yet.
If you e-filed your return already, it'll stay in Pending status until the IRS starts processing the backlog of returns (1/29/18).
After January, e-filed returns generally sit in Pending status for 24-48 hours before coming back as either Accepted or Rejected. Sometimes state returns stay in Pending for several days or even longer.
If you contact the government or try look up your refund status while your return is still pending, don't be surprised if they tell you "no record found." This is normal; until the government starts reviewing and processing your return, they won't have any record of it.