Dynamic typing
A language feature where the kind of each piece of data gets checked while the program runs. Python and Ruby work this way.
What it tells you about a developer
This carries no negative reading on its own. A developer whose dynamic-language projects carry real test suites is demonstrating the discipline that typing would otherwise enforce.
Dynamic typing makes code faster to write, and it is standard in scripting, data exploration, and early-stage product work. It is a design choice with real advantages. Most professional teams pair it with tests, or move the busiest parts of a codebase to typed tooling as the codebase grows.