Question

what is meant by interfearing anions

Answer 1

 these are also spectator ions. spectator ions don't participate in the actual chemical reaction. 
Suppose we have this reaction: 
NaOH + HCl --> NaCl + H2O 
If we separate each substance into their component ions (with the exception of water), we will get the following: 
Na+ + OH- + H+ + Cl- --> Na+ + Cl- + H2O 
the spectator ions don't participate in the actual reaction, and they appear in both sides in the chemical reaction. The spectator ions are Na+ and Cl-. Removing the spectator ions gives us the net ionic reaction 
H+ + OH- --> H2O 

Answer 2

 these are also spectator ions. spectator ions don't participate in the actual chemical reaction. 
Suppose we have this reaction: 
NaOH + HCl --> NaCl + H2O 
If we separate each substance into their component ions (with the exception of water), we will get the following: 
Na+ + OH- + H+ + Cl- --> Na+ + Cl- + H2O 
the spectator ions don't participate in the actual reaction, and they appear in both sides in the chemical reaction. The spectator ions are Na+ and Cl-. Removing the spectator ions gives us the net ionic reaction 
H+ + OH- --> H2O