Asio is a cross-platform C++ library, see https://think-async.com. It provides a consistent asynchronous model using a modern C++ approach.
Please refer to the original asio documentation at https://think-async.com/Asio/Documentation. Asio also comes with a number of examples which could be find under Documentation/Examples on that web site.
ESP platform port currently supports only network asynchronous socket operations; does not support serial port. SSL/TLS support is disabled by default and could be enabled in component configuration menu by choosing TLS library from
mbedTLS with OpenSSL translation layer (default option)
SSL support is very basic at this stage and it does include following features:
DH property files
Certificates/private keys file APIs
Internal asio settings for ESP include
EXCEPTIONS are enabled in ASIO if enabled in menuconfig
TYPEID is enabled in ASIO if enabled in menuconfig