Welcome, as the word suggested is a small period of time (usually 10-15min) which welcomes the children into their day.

Typically we keep any one material and let the child's own imagination take over. The child explores the material is many different ways without trying to reach any goal.

This free play with a new material opens up the mind, prepares it for the day ahead and importantly gets the group together (with the material exploration being the center).

Materials like newspaper, blocks, mask, puppet, dupatta, resource material, colors, safety pins, waste tissue rolls, etc are kept and with ZERO instructions children are allowed to explore. 

Since welcome gives children something to ponder upon, it arouses curiosity, gets them to DO - its a greeting that makes them feeling of being special and belonging.