A meeting for worship is what the Religious Society of Friends (or "Quakers") call their church service.
Unprogrammed worship is based on silence and inward listening to the Spirit, from which any participant may share a message.
In unprogrammed meetings for worship, someone speaks when that person feels that God/Spirit/the universe has given them a message for others.
They have programmed worship which includes many elements similar to Protestant services, such as a sermon and hymns.
Many programmed meetings also include a time during the service for silent, expectant waiting and messages from the participants.