Proprietary software is the opposite of free software. Proprietary software is restrictive, the source code is typically closed and not publicly available.
Proprietary software tends to have with artificial restrictions limiting what the user can do with it. A lot of proprietary software is akin to malware.
"Developers today shamelessly mistreat users; when caught, they claim that fine print in EULAs (end user licence agreements) makes it ethical. (That might, at most, make it lawful, which is different.) So many cases of proprietary malware have been reported, that we must consider any proprietary program suspect and dangerous. In the 21st century, proprietary software is computing for suckers."