From Teflpedia

To (/tu:/) is an English word that can function as a preposition or a particle.[1] It is the third most common word in English, after the, and various inflections of the verb be. The prepositional and particular usage are fundamentally related.

As a particle, it's used to form the to-infinitives of verbs.

As a preposition its primary meaning is "in the direction of". For example, "We're going to the swimming pool later". To can serve as the head of a preposition phrase, known as a to phrase.

To forms the head of the double prepositions "into", "unto", "up to".

Pronunciation; there is a weak form /tə/.