MIDP is the Mobile Information Device Profile. It is a Java API that is available on Java enabled mobile devices, so that you can write Java midlets (like applets but for MIDP) and have them run on any Java enabled mobile device. That is the dream, but it is mostly true.
MIDP 2 is the second version of the MIDP specification. There are still lots of devices that are MIDP 1. You can read more about MIDP on Wikipedia.
One of the most major differences between MIDP 1 and MIDP 2, with regards to MidpSSH, is that MIDP 1 did not require network socket support. It only required HTTP support. This is currently a problem for MidpSSH, read more about socket support.
Apart from socket support, which may prevent you from using MidpSSH altogether. There are some other differences in MidpSSH between MIDP 1 and 2.
MidpSSH builds for MIDP 2 add functionality over MIDP 1. Fortunately the functionality is mostly cosmetic: