What does “slim odds”mean?

Odds are a numerical expression, usually expressed as a pair of numbers, used in both gambling and statistics. In statistics, the odds for or odds of some event reflect the likelihood that the event will take place, while odds against reflect the likelihood that it will not. In gambling, the odds are the ratio of payoff to stake, and do not necessarily reflect exactly the probabilities. Odds are expressed in several ways (see below), and sometimes the term is used incorrectly to mean simply the probability of an event. Conventionally, gambling odds are expressed in the form "X to Y", where X and Y are numbers, and it is implied that the odds are odds against the event on which the gambler is considering wagering. In both gambling and statistics, the 'odds' are a numerical expression of the likelihood of some possible event. - Wikipedia

The definition you link to regarding low and high odds looks like it relates to betting odds. These are said to be expressed conventionally as odds against something.

In conversational usage (outside the technical terminology used in betting), however, odds are normally taken to be odds for or odds of unless the context clearly indicates that they are odds against.

You ask:

1) Is “odds” in the “slim odds” definition used as a synonym of chance? If so, is that a correct usage?

2) What does “slim odds” really mean in the end?

The synonymous phrase "poor chances" strongly constrains "low odds" to be the common (non-technical) usage of odds for or odds of. So "low odds" would mean that the event of interest is less likely to happen than not. This is consistent with the rest of the definition you've quoted regarding "slim odds".