Learn Hardcore Russian: Осторожно, кипяток очень горячий: держи кружку двумя руками. - Careful, boiling water is very hot: hold the mug with both hands.

Yes—Осторожно can function as a standalone warning meaning Be careful / Caution. The comma separates this warning from what follows (the reason/explanation). Similar to English Careful, it’s hot.

What exactly does кипяток mean? Is it just “hot water”?

Кипяток literally means boiling water (water at/near boiling point). In everyday speech it’s also used more loosely for very hot water (e.g., from a kettle or dispenser), even if it’s not actively boiling.

Why is it кипяток очень горячий (masculine горячий)?

Because кипяток is a masculine noun. Adjectives agree with the noun in gender/number/case:

кипяток (masc. singular, nominative) → горячий (masc. singular, nominative)

Isn’t кипяток already hot? Why say очень горячий?

It’s not “wrong”—it’s emphasis. Russian often reinforces warnings with extra intensity: very hot. In safety contexts this can sound natural, even if it feels redundant in English.