A standard is a basis for comparison. Standards are made either by many people that agree on something, or if some organisation makes it so. There are many different standards in many fields of daily life. Standards are important so that correct comparisons can be produced.
Other standards make things work together. For example, rail cars with a standard gauge can run on tracks that follow the same standard. Communication protocols such as Morse code and USB are standards that allow different machines and people to communicate.
The International System of Units is a standard that is agreed on by most countries. In it there is metre (or meter). It measures length. A rod (made from a special material) is in Paris. The length of the rod said, years ago, how long a meter is. When people wanted to measure something, they compared it to the length of the rod. For practical purposes, the length of the rod was divided by 10 (to get dezimeter), by 10 (to get centimeter), and so on.
The iridium rod as it is called is the original measure of meter. Now new ways of measuring the meter have been utilized such as the measuring the wavelength of radiation from a certain chemical element to determine an exact meter.
A standard in music is a song that remains popular, and is performed and recorded by many different artists. Such songs include "Yesterday" and "Something", written by members of The Beatles. Many of Hank Williams's songs have become standards in country music.