Learn Finnish: Tämä tuote on varmasti hyvä. - This product is certainly good.

It comes from the adjective varma (certain or sure). To turn an adjective into an adverb, which is similar to adding "-ly" in English, you add the suffix -sti. This gives us varmasti (certainly).

Could I just use the word varma instead?

No, because varma is an adjective. If you used it here, you would be saying "This product is certain good," which isn't correct. You need the adverb varmasti to describe the situation, just like "certainly" in English.

Why is the adjective hyvä in its basic form rather than the partitive form (hyvää)?

Because tuote (product) is a countable, singular noun. When describing a singular countable subject in an "A is B" sentence, the adjective stays in the basic nominative form (hyvä). If the subject were an uncountable mass noun like water or coffee, you would use the partitive form hyvää.

The word tuote is the subject here, but what is its stem if I needed to add an ending to it?

Tuote belongs to the group of Finnish words ending in -e. When adding endings to these words, the final -e doubles to -ee- and triggers "strong" consonant gradation. So the stem is tuottee- (for example, tuotteessa means "in the product").