In "A Poison Tree," this line literally means that the enemy snuck into the speaker's garden. The word "stole," for example, means to sneak or creep into a place. However, there is also a deeper meaning here: by using the word "stole," a word suggestive of stealing or theft, the speaker implies that the enemy entered without his permission. So, in one respect, the enemy is like a thief or a criminal.
In the next line, the reader learns how the enemy was able to sneak into the speaker's garden:
When the night have veiled the pole.
So, the enemy entered the garden at night, using the cover of darkness as a sort of protection against being detected by the speaker. In fact, it was so dark that night that the "pole," a reference to the North Star, could not be seen. Thus, the night sky helped to camouflage the enemy.