"Sweat of the brow" refers to the effort put into something, and any value created as a result.
If you work hard at something, you sweat. Some translations of the book of Genesis in the Christian Bible or Jewish Pentateuch have God telling Adam that as part of Adam's punishment, he will have to produce his food by the “sweat of his brow”.
In copyright law, the logic runs as follows: someone who has invested a great deal of time and energy in producing something needs to be protected, otherwise someone else can take it (by copying) and reap all of the benefit with none of the labor.
This is the “labor theory” of property, historically associated with John Locke. However, most copyright regimes do not grant copyright in something simply because it is the result of hard work. There is typically an originality requirement as well. The United States has explicitly rejected the sweat of the brow theory, in the case Feist Publications v Rural Telephone, which dealt with the partial copying of a telephone directory.
That being said, the EU grants protection in factual databases on what is essentially a “sweat of the brow” theory.