I'm having problems understanding IQueryable<T>

So I'm trying to understand IQueryable<T>. A tutorial I'm reading suggests using it but not really sure why. The code simply returns some values using LINQ to SQL. I've done this plenty of times in the past, but not using IQueryable<T>

Why use it with my functions that return more than 1 value?

Here's my code:

public IQueryable<Items> GetItems()
    {
        return from item in db.Items
               where item.IsActive == true
               orderby item.ItemNumber
               select item;
    }