In simple words ..Transistors are voltage barrier devices ,to operate they require proper voltage on the input(base)and on the output (colector). If you feed a small signal the transitor will NOT work . In order to get it "STARTED " you have to get it to the proper volatge level on the INPUT .That's biasing .the signal voltge is added to that BIASING voltage .and it depends of course on how you want to operate the device .