Translation:He speaks very fast. I do not understand.
It is a particle. It basically shows that someone is able to use a skill. You'll find it used after the verb (Verb + 得). [Disclaimer: 得 has other uses, so look out for them in the future.]
做 得 很 好 - zuò de hěn hǎo - do 得 very well
说 得 太 快 - shuō de tài kuài - speak 得 too fast
开 得 很 快 - kāi de hěn kuài - drive 得 very fast
If you go to this website: https://www.mdbg.net/chinese/dictionary?wdqb=hello and paste in 得, you'll see the three different meanings/uses for this character.
Semantically speaking, shouldn't it translate to: "He speaks very quickly, I do not understand." "Fast" isn't an adverb, but in the answer it's referring to "speaks". I understand that the literal translation of "快" is "fast" but in the context of this answer, isn't it grammatically incorrect in English?