L1 cache is the smallest and fastest memory that sits directly on your processor chip. It holds copies of data and instructions your CPU needs right now, so it doesn't have to fetch them from slower main memory.
Every modern CPU has L1 cache split into two parts: instruction cache (for program code) and data cache (for values being processed). Typical sizes range from 32 KB to 64 KB per core. Because it's so close to the processor, data stored here can be retrieved in just a few clock cycles, whereas main RAM takes dozens of cycles.
Think of it like keeping your most frequently used tools on your workbench rather than in a storage cupboard across the room. When your processor needs to calculate something, it first checks L1 cache. If the data isn't there, it looks in the larger (but slower) L2 cache, then L3 cache, then main RAM.
Real-world example: When editing a photo, your CPU might store the current pixel values in L1 cache so it can apply filters quickly without constantly retrieving them from RAM.
When buying: L1 cache size matters less than total cache (L1 + L2 + L3) because L1 is tiny on purpose. Look at overall cache size and core count together. A CPU with more L1 cache per core will handle single-threaded tasks slightly faster, but L3 cache size usually has more impact on real-world performance.