In the Preventive Maintenance application, "Generate Forecast" allows to see what Maximo plans to do with a given PM record in the future. You have to give it a parameter of how far into the future you want to look, and you can give that either by a certain number of days (e.g., show me the next 365 days), or by providing a fixed date (e.g. show me everything up to Dec. 31, 2024). You provide one of those, Maximo will calculate the other. Then it figures out which dates it thinks it should generate Work Orders, the Job Plan it would use when generating the Work Order on that date (which is extra important if the PM has Sequenced Job Plans), and if the Job Plan has Planned Resources, it will show you what it expects to need, when, and what it will cost.
In short, the purpose of the Forecast is for planning. Also, if necessary, you can "hard-code" some exceptions to this plan. For example, suppose you have a monthly task to do on the 1st of the month at a particular location, but that location is going to be down for 3 months (let's say January, February, and March 2024) for renovations. You could generate the forecast at least up to the beginning of the renovation (January), then change the January 1 date to April 1 -- so it won't generate that Work Order for the 3 months you don't need it.
The most important distinction though is that 'Generate Forecast' is NOT creating Work Orders. It's showing you when the schedule predicts they will be due to generate. The "Generate Work Order" option actually creates Work Order records, based on when the schedule thinks they are due, and whatever record filter you have at that time. That is, if you have one record open, it would generate Work Orders, if they are due according to the schedule and the Status is ACTIVE, for that one PM. If looking at several records from the List tab, it would generate Work Orders for all those showing in the list, presuming they are due and are ACTIVE.