Úväzok (employment rate) is the amount of working time in a contract. Plný úväzok (full-time employment) is about 40 hours per week; skrátený úväzok (part-time employment) is a reduced rate (for example, half), with a proportionally smaller salary.
In job offers, the employment rate affects salary, vacation, and contribution thresholds. Always check the employment rate in your contract, especially if you are looking for a side job or, conversely, a guaranteed full income.