This section of the application form is very important, as employers will use it to assess how suitable you are for the role, and how well you meet the person specification/recruitment profile. There may also be some additional questions in this section (see below for further guidance on how to approach these).
https://peoplefirst.nhsbt.nhs.uk/People First - Document Library/OWD/Guidance - Completing Application Forms.pdf
Please explain the grammatical form and function of "how" in "how suitable" and "how well"

Please consult a dictionary for a definition of "how." I used Merriam-Webster. But there are several good dictionaries from which to choose.

How - adverb

1a. in what manner or way
2: to what degree or extent

how suitable: Either definition 1a or 2 is fine, though I would choose 2. As an employer, you want to choose the person who best meets your needs. That person has the highest degree of alignment to your wants and needs. That is why I would choose 2.

how well: Definition 2 is probably best. Same reason as above.