As it is stated at the University page for incoming Erasmus students, registration with Foreign Police Department (Olšanská 2176/2, Prague 3) is mandatory. The deadlines depend on if you are EU/EFTA citizen or not.
When registering, you have to fill in Registration card, provide your travel document (passport or ID card), evidence of your health insurance, 1 photograph. and Letter of Acceptance issued by Charles University. Not registered students may be fined up to 3 000 CZK. Students staying at the CU dormitories are registered by the Accommodation office.