If you are using a passive instrument, such as a guitar or passive bass, a buffer is arguably one of the most important pedals to get a good sound.
Passive instruments typically provide a high-impedance output. This means that; as the length of the cable(s) between your guitar & amp increases, your tone quality decreases. Typically; if you have a very long cable or run a lot of pedals, you lose some of your high frequencies and get a warmer sound; losing your brightness.
This may be desirable for some musicians targeting a vintage tone. However; most of us would like to preserve the original tone of the guitar and make purposeful adjustments on the amp.
That’s where the buffer steps in. A typical buffer will transform your high-impedance signal to a low-impedance signal; which travels relatively effortless among pedals and long cables, reaching the amp without dramatic losses. Thus, you end up hearing the original tone of your guitar; preserving the high frequencies.
Playing passive basses, I put the buffer at the beginning of my signal chain. That way, my tone doesn’t get dimed by following pedals. Typical active instruments provide low-impedance signals and don’t really need a buffer.
A buffer will typically have a high impedance input (1MΩ) and a low impedance output (<1KΩ). If you have an always-on pedal (such as a compressor) with similar values at the beginning of your pedalboard, using a buffer could be obsolete because the compressor would be buffering your signal anyway.
Mind you that using a buffer is one of many steps to get a good bass tone.