A PSU (power supply unit) is the component inside your computer that takes alternating current (AC) from the wall socket and converts it to direct current (DC) at various voltages needed by different parts of your system, such as your motherboard, graphics card, and storage drives.
PSUs are rated by their maximum power output, measured in watts. A gaming PC with a high-end graphics card might need a 750W or 850W PSU, whilst a basic office machine may only require 400W. The actual wattage you need depends on your processor, graphics card, and how many drives you're running.
Quality matters here. A cheap PSU may deliver less power than it claims, run hot, or fail suddenly and damage your hardware. Better units carry efficiency ratings (80 PLUS Bronze, Silver, Gold, or Platinum), which mean they waste less energy as heat and run quieter. Modular PSUs have removable cables, making it easier to build and keep your case tidy; semi-modular units have some fixed cables.
Real-world example: If you're building a PC with a high-end GPU and CPU, you'd pick an 80 PLUS Gold 850W PSU from a reputable brand. It will deliver stable power, stay cool under load, and last several years without fuss.
When buying, check that the PSU has enough watts for your components (add 20-30% headroom), look for an 80 PLUS efficiency badge, and buy from established makers with good warranty support. A good PSU is cheap insurance against losing your entire rig to a power failure.