By adding an individual applicant profile, you will apply for grants as a specific person and not on behalf of an organization. If you work on grant applications on behalf of a company; state, local or tribal government; academic institution; or other type of organization, then you are not an individual applicant. Individuals do not need a DUNS Number to add a profile and submit applications. The system will generate a default value in that field where applicable.
With this different eligibility also comes several differences in the user experience with Grants.gov. Since you are the only person working on the application, you do not need to perform organization or role management functions.
The Grants.gov Online Help frequently refers to the specific roles and access levels throughout the help articles in order to explain specifically who can take which actions when. As an individual applicant, the core actions for applying and submitting applications are the same as performed by organization applicants, but there are minor steps which you will not need to take since you do not apply for grants as a team or manage other users' access to applications.