How to transfer text from one form to another one

How to transfer text from one form to another one

I know that with the usage of lookup field I can create relationships between the forms. We can create checkbox, radiobuttons etc. My question is:

If I have a form with two fields ROOM(single line) and DESCRIPTION(single line) with the following data:
ROOM          DESCRIPTION
single       This is an amazing room
double      I love this room

then I want to display them in a new form dynamically:
with the usage of a lookup (dropdown), I can choose the type of the room
but my problem is than I dont know how to display the description in a multiline (for example) field.
In other words I want in the new form when I choose DOUBLE for example from the dropdown menu then automatically to display the description.