Police certificates are needed to prove that you do not have any problem with your country laws.
It is required one certificate for you and each family member older than 18 that will travel with you.
Note: If you lived in another country for more than 6 months in a row on the previous 10 years, you will need a certificate from that country police as well.
Police certificates might take a long time to be done, so you are encouraged to create one on the moment you create a profile. On the other hand, it cannot be more than 6 months old when you submit your application.
Each country has different entities and processes. Please visit IRCC website for instructions on how to get one for several different countries.
Any document that is not in english or french must be translated by a certified translator, including police certificates. More information here.