An infinitive verb is the plain form of a verb that is not conjugated and often has the word "to" before it. It is good to know the plain or base form of a verb, since that is the form that is typically the main listing for the word in a dictionary. You may hear the infinitive "to sit" conjugated as "sat" or "sitting," but the form of the word you will need if you care to look it up is the infinitive "sit." In standard usage, the infinitive will always be preceded by another verb.
An infinitive is often used in a sentence in combination with a conjugated from of "to be." In these examples, the subject "it" is used to make general observations:
It is going to blow up!Play Caption
It's going to boil down.Play Caption
It is not enough to obey Big Brother.Play Caption
"The world is watching. It's time to detox."
-Greenpeace: Detox How People Power is Cleaning Up Fashion
The infinitives are written in bold above: to blow up, to boil, to obey, and to detox.