Michael's answer is basically correct and so is agc's, but if you want it down to specific policies/issues, Biden's approval rating has sank on Afghanistan, immigration, Covid-19, infrastructure bill. E.g. on Afghanistan:
Forty-three percent say they approve of his handling of foreign policy overall, and only 34% approve of his handling of the situation in Afghanistan. Even among Democrats, only 54% say they approve of Biden’s handling of Afghanistan. Just 10% of Republicans say the same.
On the rest:
Compared to an August ABC News/Ipsos poll, public approval of how Biden is handling key issues -- the pandemic, immigration and the situation at the U.S.-Mexico border, gun violence and even rebuilding the nation's infrastructure, the issue he's pushing this week -- is on the decline. Dissatisfaction among Republicans and independents is fueling the decline, but the president's ratings are also hampered by more lackluster approval among members of his own party than presidents typically enjoy.
Worth noting on that angle that Afghanistan is also related to immigration in some ways.
On none of these was he able to claim an unmitigated success. To pick one example, the vaccine mandates have proven unpopular with the unvaccinated, even among traditional Dem. strongholds like Black voters; the handling of Afghanistan has apparently had less impact on this demographic than the vaccine mandate:
Additionally, the rising inflation and transportation-related shortages have dinged his approval rating on the economy.
47% of Americans approve of how Biden is handling the economy, down from a high of 60% in March but similar to where it stood in August.