"What is the RootOf?" See please the corresponding help page, ?RootOf. Maple uses it to represent the solution of an algebraic equation when it fails in expressing it in terms of known functions. In your example, the RootOf enters the area when isolating Q(t).

In general, you can remove the RootOf of an algebraic expression (regardless of it being a solution of a DE) using the DEtools[remove_RootOf] command; but there is no magic here: by doing that you will not have Q(t) isolated anymore.

Independent of all that it seems to me you are using square brackets, like [...], to represent order of precedence in operations, while in Maple [] mean something else. For precedence of operations you need to use rounded brackets, like (...). If you fix you input in this way you receive a clean and explicit answer free of int or RootOf, as in:

> dsolve({de,ics},Q(t));

                                 50000               
           Q(t) = -----------------------------------
                            /               (1/10)  \
                            |       /    1 \        |
                            |120000 |W + --|       t|
                            |       \    10/        |
                  2 + 23 exp|-----------------------|
                            |           2           |
                            \          P            /