An important feature of Darwinian evolution is that it operates at the level of the individual. There is no mechanism for natural selection to change the species as a whole, other than through the accumulation of changes that lead to the survival of the fittest individuals.
The rate of evolution varies enormously between different types of organism and different environmental circumstances. It can proceed very quickly when the pressure is great, as, for example, with bacteria exposed to antibiotics, when drug-resistant mutations may arise and spread through the bacterial population within months.
Why does it matter? Evolution is coming under renewed assault, particularly in the US, from fundamentalist Christians who want creationism to be taught in schools. Although evolution has had virtually unanimous support from professional scientists for at least a century, polls show that American public opinion still favours creationism.
The idea that people today can question evolution is beyond amazing to me. It is much easier to understand some people thinking you would sail off the edge of the earth 500 years ago than anyone in the USA thinking there is any serious debate about evolution (there are parts of the world where the educational system does not give everyone a chance to see the available evidence, so I can forgive some in the world for being ignorant – not having been exposed to the evidence). And I guess there are parts of the USA educational system that are nearly so poor also where a gullible student could not see the truth. But in the USA the evidence is easily at hand – you have to intentionally remain ignorant to somehow not understand the truth of evolution.