I would like to know characteristics of this wild flower that I found this year in my yard.
Flowers are smallish, less than 1 cm diameter.
Location is Serbia, altitude 100m.
That's garlic mustard (Alliaria petiolata). It's native to Europe and western Asia (as far east as western China). It's invasive in North America. You can confirm the identification by smell - crush a leaf between your fingers, and it should smell like garlic. It's in the mustard family (Brassicaceae). Hence the name.
Garlic mustard is edible, and quite tasty. The leaves of the young plants (before they develop tall flowering stems) make the best pesto, but you can also use the older plants in the growth stage shown here. The older plants are somewhat bitter, so they may go best if you combine them with a more neutral green like spinach or arugula. You can also use the leaves as a cooked green by themselves or in soups and sauces.
You can add a few leaves to a green salad if you like a pungent raw garlic flavor. The flower clusters also make an attractive and tasty addition to a salad. I've heard that the seeds make a delicious, extremely spicy mustard, but I can tell you from experience that they are too hard to grind in a blender or food processor.