A lullaby is a song which is sung to help a baby or small child go to sleep. Composers of classical music often wrote pieces for instruments to play (very often for piano solo) which they called "lullaby". They also used the French word Berceuse. Brahms wrote his famous Wiegenlied originally for a young singer, Bertha Faber, when she gave birth to her second son.
Lullabies always have a gentle, rocking rhythm with a simple accompaniment. Frédéric Chopin's Berceuse is a lullaby for piano. Gabriel Fauré wrote one at the beginning of his Dolly Suite for piano duet. There are many other examples.
Mothers sing lullabies to their children at bedtime. Rock-a-bye Baby and Hush Little Baby are two very well-known lullabies.
There is an example of lullaby:
On the tree tops,
When the wind blows
The cradle will rock.
When the bough breaks
The cradle will fall,
And down will come baby
Cradle and all.