LUKS mitigate brute force attacks even if salt parameter is know?

From What users should know about Full Disk Encryption based on LUKS

In Linux world, LUKS implementations are based on cryptsetup and dm-crypt.  In  order  to  mitigate  the  problem  of  brute  force  attacks based  on  weak  user  passwords,  LUKS  combined  the  ideas  of  salt and key derivation function (i.e., PBKDF2). Because salt parameter is known and user password may be guessed, we focus on iteration counts and their ability to slow down a brute force attack as much as possible 

I can’t understand how can the usage of salt (when it is known as in this case) mitigate brute force attacks ? What am I missing ?