Hold is a family of modules. All does the same with different types.
By default if an output of a node is used then that specific node will be executed/evaluated in every frame.
There are cases when you don't want this, because it is expensive to calculate or you just want to freeze the current value. (for example an image from a camera)
In these cases you can use Hold.
The Out of hold will not change unless one of these happens:
1. Pass trigger is triggered
2. Pass If logical value is set to On
3. Period amount of frame passed and Pass Periodically is On
Pass Periodically by default is On so you probably want to turn it Off.
With Force you can force the system to execute/evaluated the input pin even if the value itself will be dropped. This can solve some issues if it is a problem if a node is not execute/evaluated in each frame.
However in most cases the best if you turn it Off.
Previous output is the previously passed value. Passed is triggered when Pass is triggered or when Pass If became On.
So basically in programming terms, this is equivalent to a mechanism to call functions on demand, since everything else is running in the main routine every frame.
This is really essential to provide manually or periodically triggered functionality.
Fantastic.