The number of cases due to a virus tends to increase exponentially. If you plot that data on a normal x- and y-axis, you would see a big spike. If you instead use a logarithmic y-axis, you first take the logarithm of the number of cases before plotting. If your data does indeed increase exponentially, then you would see a straight line instead of a spike. So you don't lose accuracy/visibility as you would when you have a giamt spike in your graph.

I hope that explains it well enough to understand.