In plain words
An application is software you use for a specific task.
Why learners confuse it
If one application fails, it does not automatically mean the whole operating system is failing too.
An application is a program people use to do a specific job.
People often call every problem an operating-system problem when sometimes only one application is failing.
An application is user-facing software designed to help with a task such as browsing, writing, drawing, or calling.
An application is a program you open to do one kind of job.
An application is software you use for a specific task.
If one application fails, it does not automatically mean the whole operating system is failing too.
Study the larger concept after the word itself becomes clear.