The ARRL's Logbook of the World (LotW) is a repository of QSOs submitted by users from around the world. When both participants in a QSO submit matching QSOs to LotW, the result is a QSL that can be used for ARRL award credit. The process for enrolling in LotW is described here.
To ensure the authenticity of QSOs submitted by a user to LotW, the ARRL requires that the file containing these QSOs be encrypted with a unique key. Each user receives their key from the ARRL as part of the enrollment process, which requires the user to submit proof of identity. The ARRL embeds this key in a Callsign Certificate, and refers to the process of encrypting a file containing QSOs as digitally signing that file.
A Callsign Certificate is itself a file, but one whose contents are not directly readable by users; Callsign Certificate files have the suffix .tq6. The ARRL provides an application called TQSL that lets you request a Callsign Certificate from the ARRL, and manage Callsign Certificates sent to you by the ARRL.
If you have always operated with the same callsign, then one Callsign Certificate is all you need. If you have operated with more than one callsign -- with your novice callsign, or portable from another region of your country, or from another DXCC entity -- you'll need to request a Callsign Certificate for each such callsign. Start by requesting a Callsign Certificate for your current (primary) callsign. Once the ARRL has verified your identity and issued your first Callsign Certificate, you can submit that first Callsign Certificate as proof of identity with your requests for additional Callsign Certificates, eliminating the need for the ARRL to review documentation before granting those requests. When submitting QSOs to LotW, group them in batches made with the same callsign; the file containing each batch can thus be signed by the appropriate Callsign Certificate.
You must also define a Station Location for each geographical location from which you have operated.
A Callsign Certificate file (.tq6) will become invalid if you copy it to an external hard drive, a thumb drive, to another computer on your home network, or to a cloud storage service like DropBox or Google Drive or SkyDrive. To protect against the loss of your Callsign Certificates and Station Locations to a hardware or software failure, direct TQSL to create a backup file.
Step-by-step instructions for configuring DXKeeper to interoperate with LotW are provided here.