This refers to the structure of the phone memory for storing information.
Shared (dynamic) memory means that there is one large "pool" of memory available for multiple types of information.
The different types of info can include photos, videos, music, voice recordings, software applications (Java, BREW, etc.), wallpapers, and ringtones. It may also include contacts, calendar entries, and to-do items.
So with shared memory, if you have a large number of music files or ringtones on your phone, that will reduce the amount of memory available for software, photos, and videos, or vice-versa.
The alternative is partitioned memory, where the memory is divided into fixed areas for each type of information. The disadvantage of partitioned memory is that if you fill up the photo memory, for example, you can't take any more photos until you delete some, even though there may be plenty of (wasted) memory available for other types of info.
Some phones have a sort of hybrid memory, where some types of info share memory, such as ringtones, wallpaper, and software, while memory for photos and contacts are partitioned, for example.