If any body purchase the share of a company whose per share 
value is Rs.10 (Rs 2 on application,3 on first allotment & 
Rs 5 on final call)and company demand the the value of 
application & first allotment i.e. Rs. 5 but person who 
purchase one share pays Rs 2 only in stead of Rs.5.So Rs.3 
will be calls in Arrear.

call in arrear is the amount which is balance on 
shareholders, who has purchased the shares of the company.
for example if a share face value is rs 10/- ( rs 3/- on 
application, rs3/-on allowtment, rs2/-on first call and 
balance on 2nd call) if allowtment is made than the rest 
amoount rs 4/- will be call in arrears.