TIM_Period is the value to be placed in the automatic reload register when the timer is configured.TIM_Prescaler is used to set the prescaler.The clock after dividing is the clock of timer.The third parameter is TIM_CKD_DIV which is not used when the clock resource is the internal clock.This information can be obtained by looking up the register description.
Clock division is used to set the sampling frequence of the filter.The filter is only used when the clock resource is an external input or an external trigger input,then,the clock division is applied.