The bass player has the most crucial role in the band. Everyone in the group depends on the bassist's subtle (and sometimes not-so-subtle) lead. If the bassist makes a mistake, everyone in the band and the audience will instantly know that something is wrong.
A bassist is responsible for linking the harmony of a song with a distinctive rhythm (groove). This link contributes to the feel, or style, of the music. Feel or style determines whether a song is rock, jazz, Latin, or anything else. Any bass player has to be able to emulate any bassist in any style and, at the same time, be creative — using own notes and ideas!
Every song is made up of chords that are special to that tune, and all the notes in the tune relate to the sounds of those chords. In some songs, all the chords are the same, and so all the notes relate to that one chord sound, making such songs easy to play. Most songs, however, have different kinds of chords in them; in these, the first group of notes in the tune relates to the first chord and has one kind of sound; the next group of notes relates to another chord sound; and so on throughout the song.
By playing one note at a time in a rhythmic fashion, you (as a bass player) propel the music along. You'll set up each chord for the other players in your band by choosing notes that lead smoothly from one chord sound to the next.
Bass lines can vary quite greatly from one genre of music to another, but all bassists still provide the same role: that of creating a foundation, often with a drummer, for the other instruments to layer their parts over (excluding some virtuoso bassist solo albums where the bass is also playing the melody). It doesn’t matter if it is a fairly prominent funk bass line or the bassist is just simply following the rhythm guitarist’s chord progression, the role is still the same.
Maintaining the Pulse
Maintaining the pulse of a song is one of the most basic aspects of the rhythm section. What the bassist plays can have a pretty vast effect on the impression a song actually gives.
Every musician should be able to keep proper rhythmic timing, not just those who learn bass guitar or drums. While it is generally much more noticeable when the rhythm section is out of time, it doesn’t excuse lead guitarists or singers from learning to properly time their parts.
The rhythm section often helps to establish many of the traits that make a song actually sound like a particular genre. This is especially true for more bass focused genres like rock, pop, funk jazz or reggae, and can often be seen in hybrid genre bands. For example, most of the reason the Red Hot Chili Peppers is considered a funk inspired band is because of the funk influenced bass lines that are quite commonly used in many of their songs.
However, it doesn’t necessarily have to be the bassist who makes those types of contributions. The Police are also a funk inspired rock band, but their bass lines were generally straight forward rock lines. It was the rhythm guitarist and drummer that gave the more funk feeling to many of the songs. It is still the rhythm section, but the bass is usually just one of the instruments contributing to the rhythm of a song.
While fairly uncommon, there are some songs out there that are simply just a bass. Not surprisingly these are almost exclusively written by virtuoso bassists with a wide enough range of techniques and musical knowledge to often play both the rhythm and melody at the same time, or at least be able to fairly seamlessly drop between the two. Sometimes they are supported by a drummer or singer, but these are usually fairly minor contributions in these types of songs.
These are almost exclusively bassists that have their bass guitar lessons completely down and then moved on and expanded to learning theory or how other instruments in a band function, but it does go to show that the bass is capable of a lot with a lot of practice and dedication.