What does "Urtext" mean?

A good example for the particular advantages of a Bärenreiter Urtext edition can be found in mm. 1ff. of Catacombae, No. 8 of Mussorgsky’s Pictures at an Exhibition. For the first time this new publication takes into consideration the placement of the dynamic markings in mm. 1ff. Mussorgsky presumably intended:

The striking dynamic markings in Catacombae do not appear to be redundant doublings, but should rather be assigned to the different voices: the sforzati in mm. 4–8 as well as the dim. in mm. 5 and 7 only affect the melodic upper voice, which should evidently stand out against the great sighing of the overall dynamics, alternating between ff and p.