Readings are ways the kanji is read/pronounced. Vocab are words that contain the kanji – occasionally, as is the case for 力, the vocab list can contain words which consist of just that kanji on its own, but usually it’s two or more kanji stuck together, for example 協力 (きょうりょく = cooperation) or 能力 (のうりょく = ability). General rule of thumb is that you use kun’yomi when it’s on its own or has a hiragana ending, and on’yomi when it’s glued to other kanji, but there’s always exceptions.

(Speaking of exceptions, my dictionary has stand-alone vocab entries for all three readings of 力. Curious.)

I was a little confused too in the beginning but,
when it comes to kanji you have two decks:
1. Kanji deck (kanji)
2. Kanji-vocab deck (words made of stand alone kanji, two or more kanji or kanji with hiragana).

What are you supposed to give as an answer?
1. the meaning of the kanji or the on’yomi of the kanji.
2. the meaning of the word and the way to write it in hiragana/pronunciation.
(if you don’t have this I recommend this post), makes anki-sama real easy!

Especially when it comes to stand alone kanji it can be confusing but at the top of the anki screen you can cheat, you can see which deck you’re in.
Now you know if it is the on’yomi they ask of the meaning and the pronunciation of the kanji. (sometimes can be both on’yomi if there is an exception > numbers).

Example:
刀　(this is what is asked)
wich deck?
kanji > it asks the on’yomi > と
kanji-vocab >　it asks the meaning and the hiragana of the whole word > かたな　、sword (katana)

I hope it’s more clear?
It’s confusing I know, sorry if I made it more dramatic -.-

Good luck!!! :D

You have to differentiate between kanji and vocab. Kanji are abstract, vocab is concrete. Kanji tells something about all the possible ways how that particular kanji can be read or what it can mean. Vocab tells us which of the readings are used to form that word. Or even better it is read.

That’s how I think about it. I think I just connected Kanji with quantum mechanics …