For questions pertaining to the positive aspects of character optimization, often abbreviated as CharOp. (If dealing with player problems relating to optimization, use the [powergaming] tag instead.)
Character optimization (aka CharOp, Char-Op) is the process of crafting a game character using the rules to make them excel at some given role. Includes "practical CharOp", which is optimization that you might actually use in a game, and "theoretical CharOp."
This tag is for these positive aspects of CharOp. If dealing with player problems relating to optimization, use the powergaming tag instead.
Read Are character build or optimization questions on topic? to understand how to ask and answer a good CharOp question here. For D&D 5e and sufficiently similar systems, see also How do I ask a good character build/optimization question for D&D 5e?.
While practical CharOp, solving specific build problems for a specific character in a specific game is encouraged here at RPG.SE, general theoretical optimization questions tend to be broad guessing games seeking things like "the best DPR." and usually are more suited to forum discussions.
Character optimization does not necessarily require being the "best possible build" at dealing damage or killing things, Instead, these questions can focus on any aspect of the character "made better" to some degree.
A completely valid question could be: "How can [this specific character] not die so much?" Another valid question is "My party [Made of this type of character, and this other type and this third type] is doing more damage than I am. Here's my build and the books we're using, how can I keep up with them?"
The essence of good optimization is finding and articulating intentions as accurately as possible. Stating the objective clearly with the parameters of the problem (disallowed books and combos) allows a good, correct answer to be found. Vague "I want to be better" while optimization, does not permit a good answer.