C++: What is the difference between std::sort and std::stable_sort?

1struct Person {
2    std::string name;
3    int age;
4};
5
6std::vector<Person> p = {
7    {"A", 30}, {"B", 25}, {"C", 30}
8};
9
10// sort — may reorder A and C
11std::sort(p.begin(), p.end(),
12    [](auto& a, auto& b) { return a.age < b.age; }
13);
14
15// stable_sort — preserves A before C
16std::stable_sort(p.begin(), p.end(),
17    [](auto& a, auto& b) { return a.age < b.age; }
18);