What does it mean 'there is no cow level'?

Solution 1:

Way back with Diablo 1, there were rumors that clicking repeatedly on the corpse of a specific cow would open up a secret level.

Blizzard denied this (rightfully so), and made a humorous nod to the rumor by including a cheat code in their game Starcraft. Typing "there is no cow level" into the console would allow you to skip a level.

In Diablo 2, they added an actual secret cow level.

In Diablo 3, they did not add in a cow level, but instead added a different secret level.

Solution 2:

If you know nothing else about video games, know these three things:

1. The cake is a lie.
2. There is no cow level.
3. All your base are belong to us.

With this knowledge, no video game meme will ever confuse you.

Solution 3:

Its referring to Diablo 2. There was a Secret Level the "Cow Level". Blizzard said in Diablo 3 will be no Cow Level.

But... in D3 are Secret Levels.