I think it meant that the yard (a square place, evidently a sort of garden, viz. the geraniums in the next sentence) was a subdivision of a larger area, maybe because they shared the garden with owners of adjacent buildings. Straight edges shouldn't be a problem for you, Kiku: the area had been divided by straight lines. The weeds had been roughly removed, whacked away - that's not a very common expression - rather than pulled up with care, meticulously.
Subdivision is the act of dividing land into pieces that are easier to sell or otherwise develop, usually via a plat. The former single piece as a whole is then known as a subdivision in the United States. If it is used for housing it is typically known as a housing subdivision or housing development, although some developers tend to call these areas communities.
Houses in Vietnam are different from ones in Western countries. So hard for me to imagine.
Yard in Vietnam is simply an empty brick or cement yard (may be regarded as a ground in mountainous area); not referred to as garden, which is a soil area growing plants or vegetables. So, I understand the literal meaning of "straight edges" but seems it doesn't match to my image of a garden ^^