Let's consider a file with these lines.

line 1 before
line 2 before
HELLO WORLD
line 1 after
line 2 after

Line(s) before

-B1 means "the match and one line Before".  

grep -B1 "HELLO WORLD" example.txt

-B1 will yeild the following result.

line 2 before
HELLO WORLD

-B2 means "the match and two lines Before".  

grep -B2 "HELLO WORLD" example.txt

-B2 will yeild the following result.

line 1 before
line 2 before
HELLO WORLD

Line(s) after

-A1 means "the match and one line After".

grep -A1 "HELLO WORLD" example.txt

-A1 will yield the following result.

HELLO WORLD
line 1 after

-A2 means "the match and two lines After".

grep -A2 "HELLO WORLD" example.txt

-A2 will yield the following result.

HELLO WORLD
line 1 after
line 2 after