Plene (or full) spelling is the opposite of defective spelling (sometimes referred to as scriptio defectiva). The letters vav and others are used as full vowels. See here or this video.
For example, the word, יוֹשֵב is different than the same word יֹשֵב. The first word is plene spelling since the vav is preserved as a vowel (a holem-vav); the second is not. Another example is תִהְיֶיןָ often found in Ezekiel where the ה of the feminine plural ending is not included.