Most engines (and all Minis) use an electric spark to start the burning of the fuel/air mixture which in turn powers the car.  If the engine is to run properly this ‘electric spark’ must happen at the exact right time. Setting your timing means you adjusting the time the plug fires relative to the position of the piston.
Piston position is defined as follows: at the very top of the cylinder the piston is said to be at TOP DEAD CENTER. At the bottom of its stroke it's said to be at BOTTOM DEAD CENTER. The crankshaft is marked in 360 degrees. At top dead center, the crank is at 0 degrees. At bottom dead center the crank is at 180 degrees. Timing is measured in degrees and most often ‘before’ top dead center. ( BTDC degrees)
It is most often measured either with a timing light (which is a "stop motion" strobe light which fires with the spark plug and "freezes" the crankshaft timing mark for observation), or mechanical setup (static timing). Your ignition timing changes as you drive. The number of degrees BEFORE TOP DEAD CENTER (BTDC) is called the ADVANCE. This is done mechanically (centrifugal advance), or by vacuum (vacuum advance),  or by a combination of these. Whatever the setup, advance increases with engine RPM and decreases as the engine goes under a load.

SYMPTOMS OF INCORRECT IGNITION TIMING
Symptoms of incorrect ignition timing are poor fuel economy, sluggish acceleration, hard starting, backfiring, or "pinging" or "spark knock". Too little spark advance will cause low power, bad gas mileage, backfiring, and poor performance. Too much advance will cause hard starting and pre-ignition.

PINGING, SPARK KNOCK, OR PRE-IGNITION
Pinging or spark knock is properly called "pre-ignition" and is often incorrectly called "valve noise" because it sounds similar to loose valves that need adjusting. The difference is that loose valves will make noise all the time, whereas pre-ignition happens on acceleration or under a load.Pre-ignition can be caused by too much spark advance or too low octane gas. If your car does this from time to time, you may not need to do anything. you might use a higher grade of gas, but that costs a lot of money! I tell people to just not let their car do it: either get on the gas or get off the gas and the "ping" will go away.