Simple Definition:

Yahweh is a term used in the Bible to refer to the Hebrew God. It is a name for God that is considered sacred and is not usually spoken out loud.
Many people pray to Yahweh for guidance and strength.

Very Simple Definition:

Yahweh is the name of God in the Bible. It is a special name for God that people use when they talk about Him.
People in the Bible believed that Yahweh created everything.