WordPress transients are a simple way to store markup in the WordPress database for short periods of time.
In effect, transients are a kind of mini-cache. They allow us to store finished markup, generated by a template, that’s computationally expensive to create — such as a custom WP Query.
“Transients” are called that because they eventually expire. When some period of time is up (or we intentionally replace or expire the transient), WordPress can rebuild that transient and store it again for some period of time.
This has the net effect of speeding up page load times. Rather than the template having to repeatedly crunch the same code, over and over, to produce the same chunk of markup, the webserver does it once — and serves up that saved markup to all subsequent users.
That’s a lot easier on the server, so it makes pages load faster and reduces errors that can be created when a webserver is under stress.