Constructive eviction is the idea that a landlord can evict a tenant without ever filing a case — by letting the property become unlivable. If essential services fail or conditions deteriorate badly enough that the tenant is effectively forced to leave, courts in many jurisdictions can treat the tenancy as ended by the landlord's conduct. In that scenario the tenant may be relieved of further rent obligations and may have claims against the landlord, though what qualifies and what the tenant must do first varies significantly by jurisdiction.
For landlords, the doctrine is the sharp edge of the warranty of habitability: ignoring serious repair issues is not a neutral act, it accrues legal risk. The practical defense is unglamorous — respond to habitability complaints promptly, document the response, and never use service shutoffs, lock changes, or harassment to pressure a tenant out, which most states treat separately and severely as "self-help" eviction. Rules differ by state and locality and change; if a situation is heading this direction, involve a local attorney early.
This is general educational information, not legal or tax advice. Rules vary by state and locality and change over time — check your local law and confirm specifics with a qualified professional.