Gross salary is the amount you are paid before deductions, and it is the figure your PAYE and UIF are calculated on. Cost to company is what the employer spends in total to employ you, which includes your gross salary plus the employer's own contributions.
Those employer costs typically include the employer's 1% UIF contribution, the 1% skills development levy where the business is above the R500,000 payroll threshold, the employer share of any pension or provident fund, medical aid subsidy, and the COIDA assessment. On a package structured with a fund and a medical aid subsidy, the gap between gross and cost to company can be substantial.
This matters most when you are comparing offers. A role advertised at R30,000 cost to company and one advertised at R30,000 gross are not the same job, and the second pays noticeably more. Ask which basis a figure is quoted on before comparing anything.
It also matters when negotiating a raise. An employer thinking in cost to company is looking at a bigger number than the one that reaches your bank account, so framing the request in the same terms tends to make the conversation shorter.