Also called: startup_cost, total_cost, cost estimate
In plain English
Every plan node has two cost numbers. Startup cost is the work done before the first row can be returned; total cost is the work to return all rows. EXPLAIN shows them as the cost=0.00..123.45 pair.
Why it matters
The two numbers can disagree about which strategy is best. A sort must read everything before emitting a row (high startup), while an ordered index scan can emit immediately (low startup). Under a LIMIT, low startup cost wins — which is why the planner tracks both and why a higher-total-cost path can still be chosen.