Lowland football is football played in an amateur way, without much organization. Várzea is a slang term for something informal, often low level, without much structure or support, whether in relation to professionals or the field.
The term floodplain soccer first appeared in São Paulo, when boys played soccer on fields made in the floodplain, that is, on the banks of the Tietê River, even before football became a sport professional.
Lowland football is that played on the fields of neighborhoods, villages and slums, which has no structure. Lowland soccer teams practically pay to play, for example, when they have championships, once that most don't have any sponsors, so players have to shell out money to get keep.
There are many players, who are currently professionals, who became known playing in varzea football, such as player Damião, for example. But this is a rare example, as most professional clubs still prefer to hire already professional players.
Generally, the floodplain soccer player is the one who uses the sport only for entertainment, who has another fixed work, and makes football just a physical activity, some with no intention of becoming a player professional.