A) what is the sea floor expansion?
b) how is magnetism affected during seafloor expansion?

Both are interconnected.

Explanation:

Seafloor spreading/expansion is the theory that came after the plate tectonics theory which tries to convey and to depict the evidence given by earlier scientists were in accordance with the seafloor which is expanding continuously and its expansion is a result of the driving geo forces from below.  

When the oceanic plate expand their expansion is cause and a consequence of the divergence or the convergence of the mid-oceanic ridges from below, hence due to which the rising magma or lava which upon coming out from these deep ridges tends to magnetise and thus the lava which gets deposited on either side of the ridges takes the poleward alignment of the earth.

This alignment causes the rocks to take the shape and form similar patterns as there already exists on earth. This was taken as one of the prime determinants of the seafloor spreading.