What does "1%" mean in ICAO Annex 14 § 3.9.9 (longitudinal slope changes)?
You are referring to ICAO  Annex 14 Volume I, Aerodrome Design and Operations. Section 3.9.9 says:


  Recommendation.— Where slope changes on a taxiway cannot be avoided, the transition from one slope to another 
  slope should be accomplished by a curved surface with a rate of change not exceeding: 
  
  
  1 per cent per 30 m (minimum radius of curvature of 3 000 m) where the code letter is C, D, E or F; and 
  1 per cent per 25 m (minimum radius of curvature of 2 500 m) where the code letter is A or B.
  


To better understand this, lets see what the previous section says:


  3.9.8 Longitudinal slopes 
  
  Recommendation.— The longitudinal slope of a taxiway should not exceed:
  
  
  1.5 per cent where the code letter is C, D, E or F; and 
  3 per cent where the code letter is A or B.
  


So: for a taxiway with a code letter D for example, a maximum slope of 1,5% is recommended. That would be 1,5 meters up or down within a distance of 100 meters. 

If a part of the taxiway in question is 1,5% uphill, and another part is 1,5% downhill, the transition from the uphill portion to the downhill part should not happen at a rate faster than 1% within a distance of 30 meters.

In addition to this, the radius of the curvature should not be less than 3000 meters. This means transitions must be smooth within that 30 meters distance.