Natural teeth have 2 parts.
The Crown (Above the Gum. The part you see)
The Root (Below the Gum)
Dental implants replace the roots of teeth.
To make a dental implant work you need to attach an abutment to the implant and then a crown on top of the abutment.
There are some dental implants that come from the manufacturer with the abutment attached to the implant. In this case only a crown is needed on top of the implant.