In a nutshell
The TC39 process refers to how the Ecma TC39 committee manages the evolution of the ECMAScript standard.
Proposed changes to the ECMAScript language must go through five stages before they make it to the standard revision.
|Stage 0: Strawman||The feature is only a suggestion at this stage so there’s no entrance criteria and acceptance signify nothing.|
|Stage 1: Proposal||The committee has reviewed the initial proposal and will devote time to examining it in greater depth.|
|Stage 2: Draft||After closer examination, the committee likes the proposal and expects the feature to be developed and eventually included in the standard (assuming stage 3 goes well).|
|Stage 3: Candidate||Solution is complete (i.e., example implementation is provided, spec text has been completed and approved) but feedback is needed from significant implementation usage before it can proceed to next stage.|
|Stage 4: Finished||The feature will be included in the next revision / ESNext.|
More details about each stage can be found here.