Taskserver is the server component of Taskwarrior, a free and open source todo list application. Upstream documentation:

Taskserver does all of its authentication via TLS using client certificates, so you either need to roll your own CA or purchase a certificate from a known CA, which allows creation of client certificates. These certificates are usually advertised as server certificates. So in order to make it easier to handle your own CA, there is a helper tool called nixos-taskserver which manages the custom CA along with Taskserver users and groups. While the client certificates in Taskserver only authenticate whether a user is allowed to connect, every user has its own UUID which identifies it as an entity. With nixos-taskserver the client certificate is created along with the UUID of the user, so it handles all of the credentials needed in order to setup the Taskwarrior client to work with a Taskserver.