Anything that's almighty is extremely powerful. In fact, being almighty gives something (or someone) an infinite amount of power.
Originally, the adjective almighty was capitalized and used to refer to God. It comes from the Old English ælmihtig, "all-powerful." You could also describe something that's almighty as "omnipotent" or even "godlike." Today people often use almighty in a sarcastic or ironic way to talk about things that exert a ridiculously huge amount of control over people: "All he cares about is the almighty dollar."