Is it correct to say to understanding like in this sentence: This is a definition that reduces history to its lowest terms and yet includes everything that is essential to understanding what it is really? If so, Is this sentence true?
Yes you can put 'to' before a Verb+ing word, because it is a participle form being used as noun, which is called a gerund. In this case, 'understanding' is also the noun formed from the verb 'to understand'. 'To' is being used as a preposition, not as part of the infinitive
Many verbs have noun forms ending in -ing.
This course aims to give beginners a better understanding of the possibilities of Photoshop.
Yes, 'really' was in the wrong place in your sentence.