Answers:
When HIV enter the body it any finds and attaches to a CD4+ cell which will normally bring it to a lymph node, or it catch a ride near a macrophage (another type of cell).
Either path it commonly ends up surrounded by a lymph node where on earth it can replicate and infect heaps CD4+ cell that are present contained by the lymph node. ("seed the lymph node")
It's a bit more complicated than that, but that's the simplified interpretation.
Hope this help