Depends on the business, but most of the 40% is employer contributions to payroll tax, health insurance, and other per-employee costs, and the 20% overhead is "general and administrative" (office manager, professional services, coffee, etc), coverage for non-billable employees like sales folks (including otherwise billable employees who are below 70% due to winning you the gigs keeping you at ~70%), and what have you.

If it isn't obvious, this is a bit handwavy; particular jurisdictions will have rates/requirements which materially influence the 140% figure, etc.