You use "Ja" when you respond to a positive question:

"Äter du fisk" - "Ja, jag äter fisk"

You use "Jo" to respond positively to a negated question:

"Äter du inte fisk?" - "Jo, jag äter fisk"

Disclaimer: I am not an expert.

From what I can gather, the Swedish Ja and Jo both mean "yes", but Ja is generally used as a simple yes, whereas Jo would be more likely used when giving an affirmation but with more detail, as well as being more insistant, in a sort of "Did not! Did too!" manner.

Example: Har de äpplen? "Do they have apples?"

• Simple answer: Ja "Yes" (They do)
• Detailed answer: Jo, men... "Yes, but..."

or

Du läste inte boken! Jo! Nej! etc.. "You didn't read the book! Did so! Did not! etc..."

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Jo is also used in response to a negative question

Example:

Är du arg? "Are you angry?"

• Ja (I am angry)
• Nej (I am not angry)

Är du inte arg? "Aren't you angry?"

• Jo (I am angry)[negating the negation]
• Nej (I am not angry)[confirming the negation]

jo =

yes; used as a disagreement to a negative statement.

Du har inte borstat tänderna, eller hur? - Jo, det har jag.

"You haven't brushed your teeth, have you? - Yes, I have."

Ja (yes) can be interpreted as an agreement with the person replied to. Jo is used instead of ja if this agreement could cause ambiguity. In the example above agreement with the person asking the question would be the opposite of a confirmation that one actually did brush the teeth. As such ja would be ambiguous. The answer jo removes the possibility of agreement with the speaker.

In northern Sweden it is however not uncommon for the word jo to be used in place of ja in all cases, at least in spoken language.

Source: https://en.wiktionary.org/wiki/jo#Swedish

I live in the north and can confirm we like to use jo as ja in every case.