What is a MD5 hash or signature ?
MD5 is a hash algorithm (like SHA-1 or CRC32) usually used to check data integrity.
A MD5 hash is typically expressed as a 32-digit hexadecimal number (this is an example of MD5 hash : 703862f5d0ee949ef9fc97c4be2dc6f5). This hash represents a signature (or footprint) of the data (data being usually a file or a text). In LiLi's case, data is the ISO file used as source to create your Live USB.
If data is modified (even by just one bit) then the resulting hash will be completely different. This is why MD5 is used to check data integrity and/or to recognize a file (filename is not important when computing hash).
This is a very basic explanation but you can read more about MD5 hash on its Wikipedia page.
If you want to know the MD5 hash of a file, you can use LiLi's MD5 Easy Hasher.exe located in Bonus folder inside LiLi's install folder (usually C:\Program Files\LinuxLive USB Creator\).