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.
The Japanese tradition is to sit on the stool in front of the faucets.
Caption 22, An apartment: in Japan
In the example above, the infinitive is "to sit." Infinitives preceded by "to" are called "full infinitives."
You can sit right here. -Thank you.
Caption 5, Jessica and Liz: in a Restaurant
In this example, the infinitive is the verb "sit." An infinitive without the "to" is called a "bare infinitive."
It's really exciting to know that I'm setting a good example for young people.
Caption 24, peta2 Interviews: Vegan Surfer Tia Blanco
You did well to tell me. We must know everything.
Caption 35, George Orwell's Nineteen Eighty-four: BBC TV Movie
In the first example above, you see the full infinitive "to know," and in the second example the bare infinitive "know."