This relates to how you respond when put under pressure. In a work context, pressure can be defined as:
“The stress or urgency of matters requiring attention, the burden of physical or mental distress and the constraint of circumstances.”
The ability to work under pressure involves dealing with constraints which are often outside of your control - these might be resource or time constraints, the difficulty of the task or having insufficient knowledge required to complete the task, or unforeseen changes or problems. Effective planning and time-management (to mitigate or allow for unexpected problems for example) can reduce the likelihood of some pressurised situations occurring, but will not eradicate them completely.
Changes and unexpected events, problems or challenges can - and do - often occur, regardless of how well-planned or organised you are. The ability to respond effectively to pressure and stress is therefore extremely important in any line of work. How you respond will vary from person to person; some people thrive on it and perform better when under pressure, others may panic or become ineffective. It is important that you are aware of how you act in such situations and what steps you can take to improve your effectiveness if necessary.