Input, output, doesn't really matter as long as you are comparing same side thing on both chargers.
Simple reason for that is that in practice, power in=power out (or pretty close)
Since you need charger of certain voltage for the laptop to work, your output voltage is set.
If you use charger with less than specified Current rating (Amps) you end up with less than specified power to laptop.
If that happens, one of the following might happen:
-Laptop doesn't get enough power and either cannot charge itself while in use or.. refuses to charge at all
-Charger takes in more than it's rated for (old one was 2A, new you were looking at was 1A) and will heat up considerably, causing a fire hazard
-Charger will simply combust and not work.