To make jelly you need three ingredients:
- acid
- pectin
- sugar

Acid fruit is "sour" or "tart". You can't make jelly from sweet eating
apples unless you add some acid like lemon juice. You can make jelly from
"sour" cooking apples or crabapples. The skin contains lots of pectin so
you leave the skin on when mashing up the apples to extract the juice. The
pectin is released when the mash is heated so it has to be brought to a
boil and simmered which also softens the apples so they can be mashed. The
harder the apples the more pectin they contain. I gather some green apples
early in the season to extract the juice to add to other fruit which don't
have enough pectin to make jelly by themselves.

Which brings us to grapes. Wild grapes are sour and contain acid but the
one's I gather don't contain enough pectin to jell so I add some apple
juice. You can also buy commercial pectin at the supermarket and add that
instead.

You can find out all about the chemistry of jelly on the internet.
You read what I learned about it on my website (see below) under "Food".
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