If you're doing a signal analysis of a circuit, you can simplify the analysis considerably if you separate the AC and DC analysis. Afterwards, if you want to get the total results...you just apply superposition (add your DC and AC results).
Thus, when we go for a signal analysis we set the value of the ideal DC voltage source to ZERO; a voltage source of 0 volts is equal to a short circuit b/w it's terminals.....b/c current due to some other source can still pass through it without any voltage drop (a simple ideal wire has the same property - thus ideal 0 V voltage source = ideal wire).
Similarly, in the signal analysis (applying superposition) we can simply eliminate (set to ZERO) the current source.... A current source of 0 A ... does not allow any current to flow through it... and is therefore said to have infinite resistance.