ketosis is a state that occurs when one burns mostly fat for fuel (due
to eating very little or eating a lot of fat and little carbs). For
example, if you were fasting, you would likely go into ketosis after a
few days. Instead of sugar, which is what all carbs convert to, we
begin to burn ketones, which of what fat breaks down to.

Ketosis means burning fat rather than sugar.  Simple as
that.  In detail, the body always has some sugar in the
blood because sugar is manufactured by the body so
ketosis is burning mostly fat rather than mostly sugar.

As fat gets burned it gets cut up into assorted chemicals.
Ketones are one of the chemicals that fat gets cut up into.
Burn lots of fat and the blood will have detectable levels
of ketones.  Ketones just happen to be easily detected with
test strips.  Cells can burn several types of fuel and
ketones are one of the types they can burn.

In or out of ketosis - Eat above some amount of carbs and
your body will switch to burning mostly sugar.  The ketones
in the blood will drop below detectable levels and you will
be out of ketosis.  Eat below some amount of carbs and your
body will switch to burning and your body will switch to
burning mostly fat.  The ketones in the blood will raise
above detectable levels and you will be in ketosis.  Since
the ketones get burned by almost every cell in the body,
the level of ketones goes up a bit and then stabilizes.
There is no danger of getting too much unless you're
diabetic just like there's no danger of blood sugar levels
getitng too much unless you're diabetic.

The skill part - Each person has a different amount of
carbs to put them in or out of ketosis.  It's mostly
genetic like height.  The amounts range from 15 grams per
day through 150 grams per day but they mostly cluster
in the 50-60 grams per day range.  It turns out that
while in ketosis the more carbs you eat the less the body
wants to be in starvation mode.  it turns out that while
out of ketosis the more carbs you eat the more the body
is able to store new fat.  So there's an exact point that
gives the best loss.  The least likely to trigger stalls
as well as the least likely to trigger regain.