Many new phones come with a memory expansion slot to put various memory cards into. These are usually MMC, RS-MMC, SD cards, Compact Flash Card etc. Adding a memory card to your phone is a bit like adding another hard drive to your computer - it gives you a lot of extra disk space to put stuff on.

It's recommended to install Mutoview Portable onto the memory card if you have the option, so you have more storage space to play with.

You can check how much space you have available by selecting [System] on the main menu.

See this for an example of memory card usage