Passport photos are the photos used on your passport to identify who you are.
From government website:
Rules for digital photos
The quality of your digital photo
Your photo must be:
clear and in focus
in colour
unaltered by computer software
at least 600 pixels wide and 750 pixels tall
at least 50KB and no more than 10MB
What your digital photo must show
The digital photo must:
contain no other objects or people
be taken against a plain light-coloured background
be in clear contrast to the background
not have ‘red eye’
If you’re using a photo taken during your application, include your head, shoulders and upper body. Do not crop your photo – it will be done for you.
In your photo you must:
be facing forwards and looking straight at the camera
have a plain expression and your mouth closed
have your eyes open and visible
not have hair in front of your eyes
not have a head covering (unless it’s for religious or medical reasons)
not have anything covering your face
not have any shadows on your face or behind you
not wear glasses in your photo unless you have to do so.