sudo apt/apt-get update error (no update)

Sometimes, I get following error or unable to lock directory /var/lib/apt/lists/ while updating the system. What does that mean?
The output in the terminal is as follow:

$ sudo apt update
Reading package lists... Done
E: Could not get lock /var/lib/apt/lists/lock - open (11: Resource temporarily unavailable)
E: Unable to lock directory /var/lib/apt/lists/

$ sudo apt-get update
Reading package lists... Done
E: Could not get lock /var/lib/apt/lists/lock - open (11: Resource temporarily unavailable)
E: Unable to lock directory /var/lib/apt/lists/


Solution.

Just do follow.

sudo rm /var/lib/apt/lists/lock

Then,

sudo apt-get update 

This will update your system.