Buffer size is the size of the block of audio data that your DAW and audio interface hand over and process at one time. It is often set to values such as 32, 64, 128, or 256 samples. It is a holding area that keeps processing stable, and it decides the balance between latency and CPU load.
What happens when you make it smaller
With a small buffer, audio is processed in smaller groups of samples, so the wait from input to output gets shorter. That is an advantage when you play vocals, guitar, or software instruments in real time. On the other hand, the deadline for the CPU to finish its work gets shorter too.
What happens when you make it larger
A large buffer makes it easier for the CPU to finish each block in time, so projects with many tracks and plugins play back reliably. The delay on the monitored sound increases, though, which can make it unsuitable for real-time playing.
Relation to sample rate
The same 256 samples last a different amount of time at 44.1 kHz and at 96 kHz. In theory you find the time for one stage by dividing the number of samples in the buffer by the number of samples per second. Check the input, output, or round-trip latency your DAW reports, not only the displayed value.
Switch between recording and mixing
When recording, lower it as far as playing still feels natural and no dropouts occur. When mixing, real-time input delay is rarely a problem, so raise it to keep processing stable. The best value changes with your computer, your driver, and how heavy the project is.
When you get dropouts
If you hear clicks and pops or playback stops, raise the buffer one step at a time and also check for unnecessary plugins and background processes. Keeping it pinned at the minimum is not the goal. How to display the current value in Logic Pro is explained in How to customize the control bar and display (Japanese).
