When you call a remote server there is always some extra latency. Transients cache the response which comes through the remote API calls and is stored in the WordPress database.
This saves API calls being made to the remote servers which helps in eaching API throttling limits. API call always has limit on how many calls we can make to the remote server within a give time period. Transients cahe data as a key, value or time period. Transients are similar to options in WordPress options and transients have a time span due to which they can be used to cache API calls.
SELECT option_name, option_value FROM wp_options WHERE autoload = 'yes';
If your application is taking long to load, you can delete the transient data and make it faster.