What is meant by intraplate volcanism?
Terms:
Intraplate volcanism - as the name suggests it is volcanism within the plates rather than at plate boundaries.

These are also known as hotspots. 

Ocean island basalt (OIB) - is the basaltic rocks associated with intraplate volcanism.  

Relation between mid-ocean ridges and hotspots:

Hotspot–ridge interaction produces a wide range of phenomena including excess crustal thickness, geochemical anomalies, off-axis
volcanic ridges, and ridge relocations or jumps.

For further study refer to this research paper.

A conceptual schematic of a ridge jump from that paper:





Image Source

DR: Dying rift
PR: Propagating rift  

There is a ridge jump towards the hotspot.  Due to the ridge jump lithosphere from plate B is transfered to plate A.