First and simple method is that simply press F1 in any of the fields.This is most commonly used.
Second is that, in every Navision window lower side of the window you can see a Help button there the system tell what is the use of that window.
Third is that, go to the
Help–>Microsoft Business Solutions Help:–>There you will get all the functional related help
Help–>Microsoft Business Solutions Help:–>There you will get all the Technical related help
Help–>What is this? :–> You click this menu then one arrow mark like cursor will appear you can click any of the form or field, then it will display help for that.