What is this plant with a bunch of small red berries and shiny spear-shaped leaves with twin leaflets at the bases?
This is Solanum dulcamara, commonly "bittersweet nightshade"*, one of the more deadly weeds one might encounter.
It's a member of the "nightshade" family, Solanaceae. You can see the resemblance in the fruit and leaves to tomatoes and peppers. It grows freely in reasonably dry soil, and especially likes somewhere that it can climb, like the fenced edge of a lawn. If you see it in flower, it will have interesting 5-pointed purple blossoms, each with a prominent yellow "beak". The berries are oblong, green at first, sometimes changing through yellow and orange before settling on the bright red in your picture.
All parts of the plant contain the neurotoxin solanine, which is estimated to be fatal in doses as low as 5 or 6 mg per kg of body weight.
It's a pretty plant, but keep the kids away from it.
*Note that it's not the only plant that has this common name.