What is a year?

A Earth year is the time it takes the earth to make one orbit of the sun.

(A calendar year is 365 days)

How long does it take the Earth to orbit the Sun?

The Earth's orbit around the Sun takes 365.24 days. A 'day' is defined as the Earth spinning once on its axis.

An Earth year is 365.24 days, or 8,765 hours, or 526,000 minutes, or 31.6 million seconds.

Why do we need to have one extra day each 4 years?

The reason is to keep the alignment of dates on the
calendar with the seasons of the year. If we didn't, then eventually, the months would flip around, and the northern hemisphere would have summer in January, and vice versa.

The Earth takes approximately 365.25 days to go around the Sun, yet our calendar year is 365 days. To fix this, we put on extra days in some years, called leap years.

Every 4 years, we have a leap year to add another day to the calendar to make up for the 4 missing quarters. In those leap years, a year lasts 366 days, and not the usual 365.