I think the infinitive form of a verb is the verb in its basic form only! To make it even simple to understand, this is the verb's version we find in a dictionary!
The infinite form of verb generally follows to (for instance, to work, to study, to eat...and so on). However, it's not always (I must learn English) the case.
On the other hand, finite verb is something that functions as the main verb - I eat everyday. Based on this, an infinite verb is the verb that is not finite and cannot serve as the main verb.
Useful note: An infinitive can also serve as an adverb, a noun or an adjective.
Good read here.