The neck of your bass guitar cannot be too straight, nor can it be too bent. I has to be just right for it to play as the manufacturer intended. What is “just right?” And how do you know if it is just right? 

The manufacturer of your instrument has their own idea about what just right is, and while you’re free to deviate from that if you like, going too far from their recommendation may result in problems. 

The way you assess neck relief, or the degree of bend in the neck, is with a capo and a set of automotive feeler gauges. 

I have to take a moment to address something you’re likely to hear from others or read on the internet. Some people, even experienced players, will tell you that measuring with these tools is not only unnecessary, but harmful to your setup goals. You have to do it “by feel” and everyone is their own special snowflake and no one can tell you what the right measurement is for you, etc, etc. 

These people are wrong. Do not listen to them. 

Yes, there is a degree of personal preference involved here, but if you don’t know where to begin you’re already sunk.