It generally refers to optimising the playability (i.e.how good does the guitar feel to you to play) by making small adjustments to the neck relief, string height, pickup height, string spacing, amongst others. manufacturers often publish default specs but there's no real definite measurements - it can vary by person.
YouTube does have some good videos on how to do these things - just search on 'setup' and whatever bass you have. You don't need a huge amount of special tools and it's hard to do any real damage.
it's all actually very simple to do yourself and you get to know what minor tweaks to do to suit your own playing style. If you're nervous about doing this, your local guitar shop should be able to advise you on someone local who can help.