What is meant by inferring latches,how to avoid it?

Consider the following :
always @(s1 or s0 or i0 or i1 or i2 or i3)
case ({s1, s0})
2'd0 : out = i0;
2'd1 : out = i1;
2'd2 : out = i2;
endcase

in a case statement if all the possible combinations are not compared and default is also not specified like in example above a latch will be inferred ,a latch is inferred because to reproduce the previous value when unknown branch is specified.
For example in above case if {s1,s0}=3 , the previous stored value is reproduced for this storing a latch is inferred.
The same may be observed in IF statement in case an ELSE IF is not specified.
To avoid inferring latches make sure that all the cases are mentioned if not default condition is provided.