かなり is an adverb that is used to convey a ‘considerable’ amount of something. It is often translated as ‘quite’, ‘fairly’, ‘considerably’, or ‘pretty’.
かなり will almost always appear directly before a phrase, or directly before a noun (in which case の will come between the noun and かなり).
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今月（こんげつ）もかなりお金（かね）を使（つか）ったね。
This month we used a considerable amount of money again, didn't we?
今日（きょう）は一日（いちにち）中（じゅう）仕事（しごと）をしていたからかなり疲（つか）れた。
Because I was working all day today, I am pretty tired.
メキシコまではかなりの距離（きょり）があるから、船（ふね）で行（い）くより飛行機（ひこうき）で行（い）った方（ほう）が早（はや）くつく。
Since there is a considerable distance to Mexico, you will get there quicker if you go by airplane, rather than by boat.
昨日（きのう）の事故（じこ）でかなりの人（ひと）が怪我（けが）をした。
Quite a lot of people got hurt in the accident yesterday.
かなり originally came from 可也（かなり）, and more recently 可成（かなり）. Both of these expressions mean something similar to ‘to become undeniable’, with the former structure using an older kanji for ‘to become’ 也（なり）.
かなり is still used this way today, highlighting that the amount/existence of something has reached a point that cannot be denied.
かなり may be seen with this meaning when carrying either positive and negative connotations.
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初（はじ）めて作（つく）ったの？かなり美味（おい）しいよ！
You made it for the first time? It's pretty good!
かなり痛（いた）いです。骨（ほね）が折（お）れていると思（おも）いますか？
It's quite painful. Do you think a bone is broken?