A long time ago, personal computers used to have a Turbo button alongside Power and Reset. Oftentimes computer cases also had a small LED display showing the CPU’s clock frequency in MHz or showing "HI" or "LO” to indicate the mode the CPU was running at.
This took place after the release of the original Intel 8086 processor that operated at 4.77 MHz. At the time, many software titles (and in particular games) were developed with a specific machine in mind and were tied to the specific clock frequency of the CPU to function.
When faster processors came out, the software ran too fast, so the “Turbo” button was introduced to slow down the PC as a compatibility layer so these programs could run normally. Some computers supported keyboard combinations Ctrl-Alt-+ / Ctrl-Alt-- for switching turbo mode on and off.
The “Turbo” button was very common in 286 and 386 PC clones, less common in 486 PCs and almost extinct by the time Pentium processors went mainstream in the late 90s.