Fixed work and fixed duration are two of three task types available in MS Project (the third one is fixed unit), and many other PM tools. These task types are used when you have more than one resource assigned to a task.
Fixed work tasks means that the amount of work performed on the task is fixed. This means that if you add resources, then the duration will decrease, but the work performed will still be the same.
Fixed duration means that the duration of the task is fixed. So, if you add more resources to the project, this means that the workload on every resource is lesser.
When we have one of the 3 parameters fixed (such as unit, work, duration) we will have more control while scheduling the project.
Always remember this formula: Duration = Work / Units, and you will understand how when you fix a variable the other two will vary in reverse proportion to each other.