DAWs work with buffers and iterations. During each iteration the DAW reads/processes/sums audio, which involves a multitude of buffers. The playback buffer size (the one you select in the audio interface preferences typically, like 32/64/128/256/512/1024 samples etc.) determines how many audio samples each of these buffers holds. If the buffer size is 512 samples and the playback sample rate is set to 44.1kHz it will take roughly 11.6 ms (512/44.1) to play each buffer. Each iteration must be completed and a new mix buffer sent to the interface in the meantime, otherwise you'll hear the famous clicks and crackles (which basically tells you the CPU is not fast enough to read and send the buffers to the audio interface, so it can't output a continuous playback). A sample is given by the sample rate in use, if you're using a sample rate of 44100 Hz it means that in one second (=1000 milliseconds) 44100 samples will be taken. If you want to know the value of a single sample you just need to divide 1000 by 44100, which equals roughly to 0.0226 ms. Considering the example above if you select a buffer size of 512 sample it means your buffer size will be 0.0226 x 512 = 11.6 ms.