What they are and why - a direct Apple quote ;
A provisioning profile is a collection of digital entities that uniquely
ties developers and devices to an authorized iPhone Development Team
and enables a device to be used for testing. A Development Provisioning
Profile must be installed on each device on which you wish to run your
application code. Each Development Provisioning Profile will contain a
set of iPhone Development Certificates, Unique Device Identifiers and an
App ID.
Devices specified within the provisioning profile can be used for
testing only by those individuals whose iPhone Development Certificates
are included in the profile. A single device can contain multiple
provisioning profiles.
Simply though:
Check for the status on both your device (iPhone/iPad) and within Xcode
