What is bias a tube amp ? Is it difficult?

I am looking to purchase a Jolida 1000rc or Jolida 302b moddified and or Cayin 88t. I am new do this game and it will be my first tube amp purchase. How hard is it to Bias the tubes? What does this do? And how often do I need to do it? Also Out of these intergrated amps which will give me the most bang for the buck? I have tyler tylos . Thanks Joe