The action refers to the height of the strings, or how far away they are from the fretboard. Lower action is usually easier to play, however, a lighter touch is required to prevent the strings from rattling. The string height can be adjusted by raising or lowering the bridge saddles. This can usually be done with an allen wrench key of the correct size. Be sure to use the correct size, to prevent stripping the screw. The neck relief can also have an effect on the action. Relief is the curvature in the neck, and is discussed below.
Neck relief refers to the bow of the neck. Fretted basses generally need a small amount of relief in order to play properly. This means that the neck will bow away from the strings, as opposed to bowing towards them. The latter, also known as a "backbow" is not desirable as this would cause the neck to have a hump in the fretboard, which would cause buzzing at certain frets. To adjust the relief, the truss rod must be rotated in a certain direction. The truss rod is a metal rod that is inside the neck underneath the fretboard. Please note that truss rod adjustments are a tricky business, and there is the potential to seriously damage a good bass. However, with caution, the adjustments can be made at home. If you are unsure of your abilities, do not hesitate to take your bass to a repairman, who can probably show you the correct way to perform the task. This disclaimer aside, you will need to obtain the proper sized allen wrench for your truss rod. (The incorrect size can strip the truss rod.) The truss rod can be accessed at either the peghead or at the neck joint. To lower the neck relief, turn the truss rod clockwise. To raise the relief, turn the truss rod counter-clockwise. It is important not to turn the truss rod too much in any direction. Never turn it more than a quarter turn at a time, and always give the neck time to adjust itself to the change. A little goes a long way in truss rod adjustments.
Setting the intonation of the bass involves adjusting the string length, so that the fretted notes are as in tune as possible across the fretboard. The first step to getting proper intonation is making sure the open string is tuned as accurately as possible. Next, play the octave at the twelfth fret of the same string. The note will probably be flat or sharp by a few cents. Using a screwdriver, adjust the bridge so that the saddles move forward or backwards. If the fretted note is flat, you will be moving the saddle towards the neck. If the fretted note is sharp, move the saddle away from the neck. After you make the adjustment, the open string will be out of tune, so be sure to re-tune it. Then check how in-tune the octave note it. Repeat the process until the octave and the open string are both in tune. Note, however, that a bass will never be in perfect tune across the whole fretboard. There will always be minor variations in certain positions. Setting the intonation as described above will minimize this effect, however.
