In progressive sentences, the verb takes the -ing form — that is, the present participle.

Literal

Progressive [possessive-の] sentence [in-では], verb [possessive-の] -ing form, that-is present-participle [result-となります].

進行形 'progressive form,' 現在分詞 'present participle' — English grammar terms. つまり 'that is to say, in other words' glosses the -ing form with its formal grammatical name. となります is formal 'becomes/is.'