To quote their inventor, Brendan Gregg, Flame graphs are a visualization of profiled software, allowing the most frequent code-paths to be identified quickly and accurately. Clinic Flame is specifically for Node.js and is built into Node Clinic. It collects metrics using by CPU sampling, then tracks top-of-the-stack frequency and creates flame graphs.
A flamegraph is an aggregated visualization of stacks sampled over time. It primarily visualizes two metrics. The amount of time a function was on CPU, and the amount of time a function top of stack. The last function to be called is referred to as being at the top of the stack. If a function is observed being at the top of stack more times than other functions, this function may be blocking the event loop. If a function is observed as being at the top of the stack in higher ratio it's referred to as being "hot".