Magnetic flux density, magnetic field strength or sometimes carelessly known as magnetic field refer to the B vector. B vector has the same treatment as E and g vector but somehow it is more difficult to visualize as it is 3D. Therefore to help us with 3D we employ your fingers to help us .
For a straight conductor carrying current- Use right hand and the thumb to point to the direction of current (velocity of positive charge) we curl the fingers to help us visualize the circular field with direction indicated by the fingers.
If two different currents flow near a point , then separately use right hand to find B1 and B2 and add them up as you would do to vectors. 
