In mathematical terms, 'odds' are the inverse of probability. Thus a 5%
probability (p=0.05) can be expressed as 9-1 against. The higher (longer)
the odds against something happening the LESS likely it is to happen.
Just to confuse matters, a 50-50 chance is called 'evens'. The origin is
betting e.g. racing, cards, cricket. Using odds makes calculating
winnings much easier. Odds are often written as if they were fractions,
which they are not e.g. 9/1, 100/30
If a thing is more likely than not to happen then it is 'odds on' rather
than 'odds against'. For example a horse may begin a race at '4 to 1 on'
written 1/4: it is considered 4 times likely to win as not and so you
have to wager £4 to win £1 (plus your stake back, of course). These are
short odds.
To summarise, long odds are greater than evens and short odds less than
evens. To lengthen the odds is to make it less likely, to shorten them is
to make it more likely. Hope that helps.