The Moon will be in some sign and some nakshatra (constellation) at the time of birth of an individual. The eighth sign from the sign of birth will be the ashtam house. When the Moon in transit comes to this house it will be chandrashtam. This happens every month. The Moon stays in this sign for about 2.33 days.
The peak of chandrashtam comes when the Moon enters the 16th nakshatra from the birth nakshatra. The 16th nakshatra is called the sanghaatik one which is roughly to be understood as malignant nakshatra.
Chandrashtam generally marks out the difficult days in a month.
Details about Ayanamsh can be found on the net easily.