what's the pass band ripple and stop band attenuation of a digital filter?
I hope the plot below helps answer your question. Typically I have seen the "passband ripple" and "stopband attenuation" expressed in dB as shown in the picture translating the magnitude of the ripples to dB using $20log_{10}$ as shown.  So the passband ripple is the amount of variation in the amplitude, within the designated passband of the filter, and stop band attenuation is the minimum attenuation level with the designated rejection band of the filter.



Here is a design example showing proper use of the ripple and rejection, along with common techniques used to get a first estimate of the number of taps (in an FIR) that will be needed to achieve the desired specifications.  These estimators have been detailed in other posts under the topic of "How many taps do I need...".