What does "to stumble" a site mean?

What does "to stumble" a site mean? I noticed someone on here is offering to stumble two sites for anyone who stumbles his/her one. I looked at the site offering to stumble and it looked good but gave no clue about stumbling. I'll kick myself if it's been around for years without me noticing.

 

Here you go dude: http://www.stumbleupon.com/

Don't kick yourself too hard