DigiD is an online account to identify yourself when you have to arrange matters online regarding the government, your education, healthcare or your pension fund. You can only get one if you have a BSN number. To apply visit the website of DigiD.
You need a DigiD to do your administration online in the Netherlands. For example your taxes, registration, applying for benefits and allowances and many others.
There are hundreds of organisations in the Netherlands that use the DigiD. These include:
Belastingdienst (Dutch tax office)
Healthcare providers (including pharmacies)
Gemeenten (city municipalities)
The Dutch police
Regional water boards