Short answer
Rahu Kalam is one eighth of the time between local sunrise and sunset, in a slot set by the weekday; on a 12-hour day it lasts about 90 minutes. With a 06:00 sunrise, Monday's falls about 07:30 to 09:00. Tradition avoids starting new things in it, such as signing, buying or setting out, while routine work carries on.
How ChiChakra reads this
ChiChakra divides daylight into eight equal parts and picks the weekday's segment: Monday 2nd, Saturday 3rd, Friday 4th, Wednesday 5th, Thursday 6th, Tuesday 7th and Sunday 8th. Sunrise uses the Sun's centre with no refraction, so times can differ from some panchangs by a few minutes.
What to avoid, and what's fine
Traditionally avoided: beginning something new, such as a signing, a purchase, a journey or a ceremony.
Fine to do: ongoing work, meals, study and anything urgent. A medical appointment or a deadline always comes first.
Day length changes with the season and your location, so Rahu Kalam is shorter in winter and longer in summer. Check the tool for today rather than relying on a printed chart.