Pitch, Roll & Yaw are the descriptions of the three determining angles for orientation in space; originally used for aircraft.

On an iPhone they're assigned as follows...

Picture credit rpappalax

There's a question on Stack Overflow with some examples of orientation in space [& incidentally show the potential issue, known as gimbal lock, with using vectors to describe these rather than quaternions, but don't worry about that in this instance]

The figures in your data - 270° & 0° - are, however, just far too tidy to be real, imo, so perhaps it is only recording general orientation so a photo app will know which way round to view the picture.

That would be 'held it upright [portrait] with the home button at the bottom, & no information on Pitch, which might have given you the direction you were facing.

I'm sure I wouldn't trust it any further than that if you are trying to extrapolate an actual facing direction when the photo was taken, even if Pitch info is also in the exif somewhere, unless it is not a tidy number.

Having checked the exifs on a couple of iPhone photos, I'm guessing that data was actually applied by a 3rd party app, not the built-in camera app.

I compared one taken by a simple 'job sheet' app & one by the Apple camera app.
The first contained no GPS or orientation data at all.
The Apple app actually has a slew of positioning data, including GPS & direction

GPS Img Direction Ref           : Magnetic North
GPS Img Direction               : 106.8389831
GPS Dest Bearing Ref            : Magnetic North
GPS Dest Bearing                : 106.8389831
GPS Altitude                    : 17.4 m Above Sea Level
GPS Date/Time                   : 2017:04:13 06:31:47.58Z
GPS Latitude                    : 52 deg 27' 26.02" N
GPS Longitude                   : 0 deg 4' 46.93" W
GPS Position                    : 52 deg 27' 26.02" N, 0 deg 4' 46.93" W

...which would be a perfect way to tell where my house is & which way I was facing... if I hadn't messed the data up a bit ;)