The subscription variables are file descriptors for uOrb topics (integers), which are marked as invalid in the constructor initializer list by setting them
They are then initialized here:
_selected_gyro are variables used to choose which gyro topic to use. There may be multiple gyros on a flight controller, each being published on their own
sensor_gyro uOrb topic.
int _sensor_gyro_sub[MAX_GYRO_COUNT] is an array of uOrb subscription handles.
_selected_gyro is used to get the data from the appropriate topic here:
Hope that helps.