Temporarily increasing the clock speed of a core above the normal sustainable maximum.
Modern CPU dies contain multiple CPU cores, shared cache, optional GPU part, memory controllers etc. All of these generate heat, usually significantly more when running at higher clock speeds.
The keep the die within thermal limits cores are limited to a maximum sustainable clock speed. This is the regular maximum speed and all cores should be able to sustain that indefinitely.
When not all cores are used however there is some thermal headroom. This can be used to temporarily clock a core at higher speeds.
Intel calls this “Intel turbo boost”. AMD calls it “AMD turbo core”.