You have been notified that your vitamin D level is low. Ideally, your level should be above 50 nmol/L, although levels above 30 nmol/L generally do not cause significant problems.
Vitamin D helps regulate minerals such as calcium and phosphate in the body, which are essential for keeping your bones and muscles healthy. Most vitamin D is made in the skin through exposure to sunlight, so levels often drop during the winter months. Some foods also contain vitamin D, including oily fish, egg yolks, meat, offal, and milk.
Symptoms of low vitamin D can be vague but commonly include generalised aches and pains.
To maintain healthy levels, you should take a regular low-dose vitamin D supplement—especially during the winter—between 800 and 2000 IU (international units) per day. These supplements are available over the counter at any pharmacy.
You can find more information about vitamin D here:
Vitamin D – NHS
Osteoporosis: Vitamin D for bones