A MD5 digest is a calculation of a fingerprint for some data, in this case probably a piece of text.
The purpose is to be able to know when the data has been tampered with or modified. Therefore it can be used as part of a secuity system.
The principle is that the digest will be different if you change one character (or byte) from the input. Because of the way the digest is calculated it should be (near to) impossible to find a possible change in document which has the same digest code. Worse would be for the digest to still be useful :-) However, if you want to be even more certain, you could also use a second digest (which uses a different algorithm).
A MD5 digest is also often used to calculate license codes. In this case, it is crucially important that the buildup of the string which is used as input is kept secret, otherwise anybody can calculate keys.
It is virtually impossable to rebuild the original input when only the digest is known. However, when the input is too short, this ecomes a lot easier. It is therefore recommended to make the input data long enough (more than 50 characters).
The form below allows you to calculate a MD5 digest for a piece of text. The digest (which is a 128 bit number) can be returned in either as a base 64 or a hex representation.