A process in operating system uses resources in the following way. 

1) Requests a resource 
2) Use the resource 
3) Releases the resource 

Deadlock is a situation where a set of processes are blocked because each process is holding a resource and waiting for another resource acquired by some other process. 

Consider an example when two trains are coming toward each other on the same track and there is only one track, none of the trains can move once they are in front of each other. 

A similar situation occurs in operating systems when there are two or more processes that hold some resources and wait for resources held by other(s). For example, in the below diagram, Process 1 is holding Resource 1 and waiting for resource 2 which is acquired by process 2, and process 2 is waiting for resource 1. 

Related Article : https://www.geeksforgeeks.org/introduction-of-deadlock-in-operating-system/