What is the meaning of..?

This is a basic question you should learn when you learn transistors. Search the web. You don't need us to spend 5-10 min typing to explain it properly to you.

Bias is a voltage or current offset. If you want to amplify an AC signal, There has to be a DC bias to allow the signal to increase and decrease.

Biasing

The transistor current-voltage relationship is different at certain current/voltage thresholds that the transistor is in. The different behaviors or "modes of operation" are different for different transistor types and happen at different thresholds. You bias the transistor to get it to work within the threshold you want to get it to behave the way you want it to.