In the world of beer, a lawnmower is generally an unchallenging beer, refreshing enough to be imbibed while doing manual labor. It's named, naturally, after the task of mowing the lawn.
This is not in any way a formal classification, and tends to be very subjective. It's most commonly applied to beers with a low malt profile, such as lagers, pils, wheats, pales, etc. Generally, the sorts of beers people like to drink in the hot sun. An optional criterion is that a person can have several without being in too great of a risk of accidentally losing a foot to the lawnmower.
Some people use the term pejoratively and others more positively. I've heard people say "I've just tried (insert beer here) recently. Uhg, that one's such a lawnmower" about as frequently as "Have you tried (insert beer here)? It's a really solid lawnmower." The most consistent usage, in my experience is amongst home brewers talking about their own beer. When they taste a new batch and refer to it as a lawnmower, they tend to mean something very specific: It didn't come out well enough for gifts, bragging about, or impressing people with, but didn't come out so horribly that they couldn't drink it or share it. When a friend brewed up a batch of the White House Honey Ale, I asked him how it was. He replied that it "wasn't even a lawnmower, at best it was a...I don't know...toilet scrubber."
Lawnmowers are a related, but distinct, informal classification from session beers. In the case of session beers, the primary criteria is alcohol content. Lawnmowers are also generally low ABV, but are additionally pleasant to drink while you're hot and sweaty. For example,Guinness Draught clocks in at a reasonable 4.2% ABV, which makes it perfectly acceptable as a session beer. However, I don't know many people that would consider it an ideal beer in stifling heat and direct sunlight.