Something has a sedative effect if it makes you calm and sleepy. Many things have a sedative effect. Some are pills, some are herbs like valerian, and some kinds of music also have a sedative effect.
A sieve is very similar to both a sifter and a colander. They have a handle, and a bowl-shaped part made from fine metal or plastic mesh.
Sieves can be used to remove lumps from powders, sprinkle powders like icing sugar over food, or strain the liquid out of something.