Planned Amount is your budget estimate.

Practical Amount is the sum of actual expenses or revenues - what has happened so far.

Theoretical Amount is what COULD/SHOULD have happened so far (if all of your transactions were spread evenly over the period of the budget) - is it the Planned Amount divided by the duration (start and end date) to show you the projected amount - if you have a budget of $500 per year, you would see the Theoretical Amount increase each month by $500/12.

https://www.odoo.com/documentation/user/10.0/accounting/others/adviser/budget.html