Busier means having more to do than before. Busier than ever, a busier week, getting busier.
It is the comparative of busy, with the y turning to i as it does in happier and easier. Being busy has become a way of signalling status, which is a reversal — leisure was the old marker of importance. Note the spelling of busy, which keeps a u nobody pronounces.
1adjectivebusierHaving more to do than before.
“The shop has been much busier since the new station opened next door.”
busier is a comparison of activity.