It means x÷3, which is pronounced as x divided by 3, that is, x is the dividend and 3 is the divisor. According to dividend = quotient+remainder of primary school, it should be two-thirds.

For example, we often say that 4 is divisible by 2, which means 4÷2 instead of 2÷4.

For another example, if we calculate that the year of Runnian is divisible by 4, then this year is Runnian, and 2008 ÷ 4 = 502, then next year is Runnian, and people born on February 29th can celebrate their birthdays! But not 4-2008!