Explain what a junior sales engineer does?

There are no strict definitions of a job title. Is the janitor a maintenance engineer? Can you be sure of the difference between an instructor and a teacher?

As a guidance, Junior, Associate, Senior at higher levels of responsibility in any field.
They can add interim like "Senior Associate" between Associate and Senior.
Junior is entry level. Typically it means no to little experience. It has to do in sales with the number of assigned accounts and the size of each account. There's also Inside Sales versus Outside Sales, where Inside is supposed to be an employee at an office, and Outside Sales, or "Field" meaning you go to clients to sell things. Marketing is strategy, Contracts is documents, Sales is the actually selling.

Engineer is an addition to tell the world that the person is a Professional, and the purpose is to be exempt from overtime pay, but the government is cracking down on the practice. It may also indicate technical abilities, that is, knowing the products as how they are used and understanding one from another in their science.

"Manager" can mean two things. It is either an actual people manager, doing the paperwork, assignments, hiring and firing, etc that any manager does, or could only be a title, as Account Manager. An Account Manager would have exclusive sales connection to one or more particular customers.
Let's say we refer to Intel as a manufacturer of CPUs and other electronic components. Intel has a Dell Account Manager, an HP Account Manager, a Lenovo Account Manager. That would make that person the main "go to" person for any issues between the companies. There can be one or more sales people also calling on Dell or HP from Intel, but the Account Manager is the person in charge of the customer relation. Instead of managing people, they manage one or more customer companies.

So, "Junior" is easy, meaning the entry level position. Sales, meaning selling a product to customers, is also pretty straightforward.

Engineer, Manager, Account, are less defined.
You may also see other terms in titles. Regional, meaning an area of some sort, where all customers are in one physical geography, and again may represent a low level or higher level job. A Regional Manager is in charge of the geography and could have individual sales people working for them. A Regional Account Manager, which kind-of sounds similar, could just be an area you are responsible to sell to.

Just as advertising has many different words to describe the same thing, a job can have a title that may imply more than it is.
They can call you a "Senior Regional Account Executive Manager" and it certainly sounds impressive, and may actually be high up in a company and well paid, but in a smaller firm may mean a lot less than it sounds.

When words do not have clear definition as related to a business, sometimes they get abused or given out to make someone feel better, or to separate people in an organization.

I think I could use easier words, but choose instead to be clear and specific and full in explanation. This allows you to read it carefully and possibly learn what it is about. Junior is not used in titles in the USA very much if at all

Junior is not used in titles in the USA very much if at all