How hard to rank a particular web page on Google’s search results for a given keyword.
Nobody knows how google ranks a web page for a keyword. Only Google knows for sure.
So SEO experts have found ways to measure the difficulty level of the keyword to be ranked. (These keyword difficulty level number can change from tool to tool)
To measure the keyword difficulty some people consider several factors such as domain authority, page authority and content quality of the competitors.
There are some free tools as well as paid tools which you can use to measure the keyword difficulty.
Sometimes keyword difficulty is called keyword competition or organic competition.