Liquid Nitrogen Canister
Liquid Nitrogen Canisters are used to take Liquid Nitrogen out of the Gas Liquefier machine. They are made by placing an Empty Canister in the output slot of that machine, when the output tank has some Liquid Nitrogen in it.
The canister can contain a maximum of 1 bucket (1000 mB) of Liquid Nitrogen. Vanilla buckets and other liquid receptacles in Minecraft cannot be used to hold Liquid Nitrogen, due to its cryogenic (extremely cold) nature. It needs special handling in a canister.
The colored bar icon drawn on the side of the canister shows approximately how full it is. There is also a tooltip which states the contents exactly.
If you have other mods installed with fluid-handling capabilities (for example, Buildcraft, IC2 or Railcraft) it's also possible to pipe Liquid Nitrogen out of the Gas Liquefier machine into tanks, and then fill Liquid Nitrogen Canisters from those tanks in the same way that you would normally fill a bucket.
A Liquid Nitrogen canister clicked on certain types of block (water source blocks, still lava blocks) will spill some of the Liquid Nitrogen which has a freezing effect.
This is the only known way to transform water into Ice in Minecraft outside of a cold biome (unless you have another mod with a way to do it).