Wet plate is named such because the plate/physical medium needs to be sensitized, exposed to light (take the picture) and developed all within about 12 minutes. The chemicals never fully dry and are, as the name suggests, wet! Because of this there is an art to the handling and required, learned dexterity to the whole process.
This was pretty great 150 years ago compared to needing the skill of painting a portrait, but due to the need of a traveling dark room (usually on the back of a carriage) it was largely abandoned in the late 1800's for a dry plate that could last over 24 hours. Oh the technology! Not to mention this process killed a good number of photographers because of ether explosions and accidental cyanide poisoning.
This video shows my first own glass plate cutting.... that broke... and is finished with an aluminum plate. The process is the same though!