Underfull hbox (badness 1226) in paragraph at lines 103--112

I am getting these messages and don’t know why! so please let me know how to fix them.

Overfull hbox (1.90002pt too wide) has occurred while output is active

 output.bbl, line 103
Underfull hbox (badness 1226) in paragraph at lines 103--112

 output.bbl, line 103
Underfull hbox (badness 4217) in paragraph at lines 103--112

 output.bbl, line 153
Underfull hbox (badness 3557) in paragraph at lines 153--156

 output.bbl, line 220
Underfull hbox (badness 10000) in paragraph at lines 220--222