Overbanking means having a lead-acid chemistry (flooded or AGM) bank oversized compared to available solar-only charging power. The solar:battery ratio might be something like 1:2.

Overbanked designs are generally found when a 'dweller wants to run large inverter loads and forgets how much power it takes to recharge the batteries fully every day. Failure to recharge fully leads to battery murder.

Adding alternator charging or shore power charging to solar can help oversized banks healthy.

Note: Lithium chemistries are not bothered by partial state of charge so overbanking is a non-issue for them.