What's the mean the word "cost" that is printed by Query plan?

What's the mean the word "cost" that is printed by Query plan?

for example this thing :
+-GROUPBY NOTHING [Cost: 479K, Rows: 1] (PATH ID: 1)
| Aggregates: count(*)
| +---> JOIN HASH [Cost: 479K, Rows: 219K] (PATH ID: 2)
| | Join Cond: ([secret])
| | +-- Outer -> JOIN HASH [Cost: 478K, Rows: 219K] (PATH ID: 3)
| | | Join Cond: ([secret])
| | | +-- Outer -> STORAGE ACCESS for p [Cost: 425K, Rows: 43M] (PATH ID: 4)
| | | | Projection: [secret]
| | | | Materialize: [secret]
| | | | Runtime Filter: (SIP2(HashJoin): [secret])
| | | +-- Inner -> STORAGE ACCESS for [secret] [Cost: 2K, Rows: 158K] (PATH ID: 5)
| | | | Projection: [secret]
| | | | Materialize: [secret]
| | | | Filter: ([secret])
| | | | Filter: ([secret])
| | | | Runtime Filter: (SIP1(HashJoin): [secret])
| | +-- Inner -> STORAGE ACCESS for wc [Cost: 58, Rows: 5K] (PATH ID: 6)
| | | Projection: [secret]
| | | Materialize: [secret]
| | | Filter: ([secret])
| | | Filter: ([secret])