Clipping is the concept of units in the came impeding each other's movements. IE if a character is surrounded by monsters they can't just run in any direction to escape.
Gamers might be familiar with the term via the "no clipping mode" done by the IDCLIP cheat in the Doom FPS games.
Clipping necessitates either killing a monster or causing it to relocate (knockback, taunt, fear, etc) to be able to run through the space they were occupying.
One major problem with the Necromancer in early editions of the game is that the Necromancer and his allies would experience clipping in respect to those minions, which meant that once he got a very large army, they would limit his movements greatly in tight quarters.
Necromancers could not scare off their own minions with Terror so the only way to clear a path would be to Unsummon them, losing the mana (and perhaps corse/item) investment.
This was especially the case back when skeletons gave 1 skeleton per point in the skill, compared to later when the number was nerfed.
This was also a minor consideration for other classes' Mercenary (or the Amazon's Valkyrie) but not to the same degree.
One of the later patches changed this, so minions would not clip their owners or their owners' allies. They could walk right through them.
This isn't the case in PVP where minions do clip PVP enemies, allowing necromancers to use them as mobile barriers while running away from melee PVP foes.