It is quite common for spells that require line of sight to strike the first model in their path. All this means is that the spell is comparable to a bullet in flight - it shoots out from the wizard and hits whatever is in the way. The wizard can still aim or direct the spell, but it won't travel through other models or scenery in order to hit its target. Fireball is the most obvious example of this kind of spell - Blast is another and far more potent version of the same thing.

Some spells strike the first unit in their path. This is basically the same thing, but the effect is broader and more difficult to aim, so the damage is spread across the unit. The Skaven spell Warp Lightning is like this.