Bit rate is a measure of how much data is used per second for audio or video. Audio is usually shown in kbps (kilobits per second), and with lossy formats such as MP3, AAC, and Opus it is the main guide to the balance between file size and how the audio sounds.
What the number means
For example, 128 kbps is a setting that uses roughly 128 kilobits per second on average. For the same duration, a higher bit rate generally means a larger file.
Relation to sound quality
With lossy compression, raising the bit rate increases the amount of data available to describe the sound. But the number alone does not decide the quality. The result changes with the codec, the encoder, the source material, whether it is stereo or mono, and the content of the music. In the end, judge by the delivery spec and by listening.
How it differs from sample rate and bit depth
Sample rate is how many times per second the sound is measured, and bit depth relates to how finely the amplitude of each sample is described. Bit rate, by contrast, is a measure of the amount of data after compression. With PCM in a WAV file you can calculate the required data from the sample rate, bit depth, and channel count, but with MP3 and AAC it changes with the settings and the content.
Constant and variable settings
A constant bit rate uses the same value across the whole track and makes the file size easy to estimate. A variable bit rate can spend more data on complex passages. Which to choose depends on the spec you are asked for, the playback environment, and any size limit. If a destination does not state what it supports, ask them to confirm.
Do not decide on the number alone
Before choosing a large number, check whether the platform re-encodes and what playback environment you need. Creating the file once from an original such as a WAV is safer than repeating lossy compression. To convert a WAV into an MP3 for sharing, you can choose the settings in the WAV to MP3 converter.
