The best way to explain these is by an example. Suppose you have an Opportunity data source that looks like this:
You can use CONTAINS to filter on a text field based on the content of the value.
This will return the number of all the opportunities that still need to be qualified. In our example the number will be 2, because of opportunity 2 and 4.
You can reverse the meaning of the filters by using != instead of =.
This will return the number of all opportunities not being small. In our example the number, returned by the formula, will be 3 because there is three opportunities, which does not contain the word "Small".
These functions are all case-sensitive. This means that it's important that you match your data's UPPERCASE and lowercase letters exactly 🤓.