convenience; handiness; usefulness
Refers to the quality of being convenient, handy, or useful. Often used when evaluating tools, locations, or services.
I was surprised by the convenience of this app.
The proximity to the station and the convenience are the appeal of this room.
It was only after using it that I really understood how convenient this app is.
Since living near the station, I've really felt how convenient it is.
不便 (na-adjective, 'inconvenient') is the direct opposite in meaning to the convenience described by 便利さ.
Derived from the na-adjective 便利 (convenient) by adding the nominalizing suffix さ, which turns an adjective into a noun expressing its degree or quality.