Γέμισε, σε παρακαλώ, τον κανατά με κρύο νερό.

Γέμισε is the singular imperative of the verb γεμίζω (to fill).

In this sentence, it means:

• Fill!

More specifically, it is the aorist imperative, which usually gives the sense of a single complete action:

• Fill the jug
  rather than
• Be filling the jug

So the sentence is giving a direct instruction or request.

Because γεμίζεις means you fill / you are filling and is a normal indicative verb form, not a command.

Compare:

• γεμίζεις = you fill / you’re filling
• γέμισε = fill! (command to one person)

So if you want to tell someone to do it, Greek uses the imperative form, here γέμισε.

It is informal singular: you are speaking to one person in the σύ form.

If you were speaking:

• to one person informally: Γέμισε
• to several people or one person formally: Γεμίστε

So this sentence is directed at one person in a familiar way.