openssl RAND_add() documentation refers to RFC1750. RFC1750 is silent on the matter

The documentation for the openssl library's RAND_add function has this to say about the entropy argument:

The entropy argument is (the lower bound of) an estimate of how much randomness is contained in buf, measured in bytes. Details about sources of randomness and how to estimate their entropy can be found in the literature, e.g. RFC 1750.

source: http://ift.tt/1PPBH3j

RFC 1750 can be found here: http://ift.tt/1MS1HVr

... but of course it is completely silent on the subject of "entropy" (a text search reveals zero occurrences of this word in the document).

So here are my questions:

1. What specifically is the entropy argument supposed to be a measurement of?
2. What is the valid range of values (the argument is of type double)?

Many thanks.