The DigiD (digital identification) is a way of providing secure access to national and local authority websites and to a growing number of other websites such as health insurers and pension funds.
The DigiD is linked to the Burgerservicenummer – BSN (citizen service number) which is issued upon registration at the municipality.
Applying for a DigiD can be done via the digid.nl website. Applicants will be prompted to choose a gebruikersnaam (username) and wachtwoord (password), and then these 2 codes are subsequently used to gain access to relevant websites.
EU/EER citizens are allowed to apply for a DigiD when not resident in the Netherlands if they already have a BSN. On application via the DigiD website they will recieve a "baliecode". Make an appointment at the local muncipality, bring this code and passport/ID, and obtain an activation code for the DigiD. Then activate the DigiD via the DigiD website.
The DigiD app is being used more often as method of access to websites, so it should also be requested when you apply for a DigiD.
Once you have a DigiD you can also use it when you are not in the Netherlands.
Here is a more detailed video explanation on DigiD if needed.