apt - Why is ubuntu unable to install two programs at once?

I tried to install two programs at once in Ubuntu via terminal. it says this.

E: Could not get lock /var/lib/apt/lists/lock - open (11: Resource temporarily unavailable)
E: Unable to lock directory /var/lib/apt/lists/ 

Could not get lock: why does /var/lib/apt/lists/lock need to be locked?

I'm so glad if anyone can explain the application installation process for me. Also something about advantages and disadvantages of using a package manager like "apt" compared to the package manager in MS windows.