This is the same red-black tree fix-up as the 3D version, drawn a completely different way: instead of a node-link graph, every node owns a horizontal slab of the key domain — the exact interval the BST invariant already assigns it (everything under its left child is less than its key, everything under its right child is greater). Depth runs down the screen; a node's slab is bounded by its parent's key on one side and the parent's own slab edge on the other, so the picture is literally the recursive partitioning of the number line that a BST performs, not a repositioned tree diagram.
Inserting a node as RED can only break the "no red node has a red child" invariant. The fix-up walks up resolving it via the node's uncle (grandparent's other child):
Case 1 (uncle RED): parent, uncle -> BLACK; grandparent -> RED; recurse up
Case 2 (uncle BLACK, inner child): rotate at parent toward the outside, fall into Case 3
Case 3 (uncle BLACK, outer child): parent -> BLACK, grandparent -> RED; rotate at grandparent — done
The bottom strip is a live bar chart of the black-node count on the path from the root to every null leaf. Red-black invariant (4) says every one of these bars must be exactly equal — that equal-height chart is a direct numerical proof that the tree stayed balanced after every rotation.
Gold-outlined slabs mark the node(s) the current step is acting on.