Understanding Node Embeddings
First, let me give you an intuitive explanation. When you drop a pebble in water, you see the ripples being formed. Imagine that in reverse. All those ripples comes together at the point from which they started.
Node embeddings are like that. You take the information of neighbourhood nodes and combine it with the information in the original node. The art lies in how you combine the neighbourhood nodes and until which k-hop neighbours you do it. 1-hop means direct neighbours, 2-hop means neighbour of neighbours and so on.
In context of GraphSAGE, the combination of information is done using a simple feedforward neural network and the neighbourhood is incorporated iteratively. See link for further understanding.