I think NOAD's second definition of chemistry defines this pretty well:
chemistry (noun)
1 the branch of science that deals with the identification of the substances of which matter is composed
2 the emotional or psychological interaction between two people, esp. when experienced as a powerful mutual attraction
This word is often used when talking about romantic couples, or potentially romantic relationships:
Linda, how come you didn't go on a second date with Rob?
Well, Heather, there just wasn't any chemistry between us.
However, the word also used outside the bounds of romantic or sexual relationships. Two athletes can be said to have good chemistry if they play remarkably well together. A student and his advisor can be said to have good chemistry if they enjoy doing research with each other, and their conversations become catalysts for new ideas. Two actors can be said to have good chemistry if they play off each other very well. For example, in a biography about hockey superstar Wayne Gretzky, Matt Christopher writes:
From the beginning there was an incredible chemistry between center Wayne Gretzky and his right wing [Jari Kurri].
and a CNN piece looking back at the successful Star Trek series mentions:
Leonard Nimoy and William Shatner shared a unique chemistry, making their on-screen performances and off-screen appearances a treat for fans.
In the context quoted by the O.P., however, it sounds like the word is being used mostly in the sense of a strong romantic attraction coupled with compatable personalities.
Whichever way the word is being used, though, two people with good chemistry tend to make each other better.
'It' is a dummy subject. English clauses which are not imperatives must have a subject. Sometimes we need to use a ‘dummy’ or ‘empty’ or ‘artificial’ subject when there is no subject attached to the verb, and where the real subject is somewhere else in the clause.
Dummy subjects (Cambridge Dictionary)
Best Answer
"Ken" derives from Middle English but, as far as I know, is chiefly a word used colloquially by the Scottish, which is probably why Google translate doesn't handle it correctly. As a (more or less) regional colloquialism you probably shouldn't use it unless you're deliberately trying to imitate people from that region, otherwise it'll feel forced and unnatural.
Also, not everyone will understand "ken" or "beyond my ken", unless they've heard it before and recognize the idiom.
I most commonly see "ken" in writing from authors who are trying to imitate a Scottish accent, "I dinna (don't) ken what yer (you're) sayin', lassie (young female), because I'm a wee (little) bit deaf in my right ear" and so on.
Note this isn't necessarily an accurate representation of how the Scottish accent sounds. It's just the way it's often popularized. If you're unfamiliar with the Scottish accent I recommend you look up videos on YouTube. There are actually many different accents from around the Northern part of the U.K., some of which are difficult to understand for anyone who isn't a local.