The metre is the SI base unit for length. It is defined by the distance travelled by light in a vacuum in 1/299,792,458 of a second.
The metre is a measure of distances. Accurate distances are useful when building large things, like bridges or buildings, or when researching small things, like viruses or proteins.
How we measure distances has changed a lot throughout history. King Edward II defined an inch as the length of three barleycorns in 1324, and in the late 18th century, a metre was defined as one ten-millionth of the distance from the equator to the North Pole. Neither of these are sufficiently accurate or practical for modern applications.
In 1875, an alloy of platinum and iridium was made and was called the International Prototype Metre. While more practical, this is still not as accurate as we can now achieve. One of the issues with physical artefacts like this prototype is that, over time, the length of can change.
Today, we can precisely define a metre as the length of the path travelled by light in a vacuum in 1/299,792,458 of a second.