Occam’s Razor states “Pluralitas non est ponenda sine neccesitate" or "plurality should not be posited without necessity."
The words are generally attributed to the medieval English philosopher and Franciscan monk William of Ockham (ca. 1285-1349).
The term ‘Occam’s Razor’ is often bandied about in debate, and sometimes used incorrectly.
Occam's Razor is not, for example, a law which states that the simplest explanation for something is always true.
It is a logical tool useful for deciding between competing explanations. It forbids the unnecessary proliferation of theories.
So if you have an explanation for something which is sufficient, you do not believe anything else until such time as it can be shown that that something else is necessary, and that the original explanation was therefore insufficient. So if x is sufficient, you do not believe x+y until it is shown that y is necessary and x insufficient after all.
For example, suppose the pavement on your street is wet. What's the explanation?
If it was raining ten minutes ago, your theory might be be: "the pavement is wet because it was raining".
As that is sufficient to explain the wetness, then you do not need to countenance any other theories unless you have to. For example, you do not need to believe a competing explanation that "the pavement is wet because it was raining ten minutes ago and a passing elephant squirted a trunkful of water onto it."
The raining explanation is sufficient, so Occam's Razor forbids the rain-plus-elephant explanation.
But if you then find out that it was just a brief shower, and see a CCTV recording of a passing elephant squirting water five minutes ago, you'll have to revise your explanation.
Occam's Razor then forbids any other explanation than "the pavement is wet because of the rain and the elephant" until such time as more evidence comes to light showing that the current explanation is insufficient.
Applying the Razor to evolution
In debates about evolution, if natural processes are sufficient to explain evolution, then Occam’s Razor forbids the explanation "evolution is the result of natural processes (which are sufficient) and also God does it", until such time as it is shown that that the natural processes are insufficient and God is necessary.
Good science applies Occam's Razor at all times and as a matter of course.