What is Programming language
The written notation developers use to give instructions to a computer, such as Python, JavaScript, or Java.
In plain terms
Every piece of software is written in one or more programming languages. Each language has its own grammar and its own home turf: JavaScript for the web, Python for data and backends, Swift for iPhones.
What it tells you about a developer
Language depth is the base unit of a developer's skill profile. One deep language plus one or two working languages is the normal senior shape. The specific languages tell you which kind of engineering they have done.
Related
Read the lesson on Programming language More in How software gets built