Boost refers to a set of third-party libraries for C++ that supplement the Standard Template Library. In many cases, Boost pre-dates many features that are later added to newer C++ revisions (like std::format and coroutines in C++20).
Boost also tends to bloat compile-times. This is because it often has large dependency graphs that the compiler must re-traverse for each #include. For this reason, Boost should never be used in cases where the standard library suffices.