To is a preposition or the marker of an infinitive. Too is an adverb meaning "excessively" or "also." Two is a number.
Apply heat to the sample.
[preposition]
They promised to write a new summary next
week.
[marker of an infinitive]
The pressure was too low.
[excessively]
The project director, too, thought it was
too low.
[also]
We can approach the problem two
ways.