Short answer
Nalla Neram (Tamil for good time) is the set of daily windows Tamil calendars mark as favourable, usually one in the morning and one in the evening, worked out from the weekday and local sunrise. ChiChakra does not calculate Nalla Neram itself yet; its Rahu Kalam tool shows today's Rahu Kalam for your city, the main slot to keep clear of.
How ChiChakra reads this
Tamil almanacs divide daylight into eight parts and mark some as good and some as difficult, alongside Rahu Kalam, Yamagandam and Kuligai. Rahu Kalam is the one ChiChakra calculates now: one eighth of daylight, in a slot set by the weekday, such as the 2nd segment on Mondays and the last on Sundays.
Using it today
Take the Nalla Neram from your family's Tamil calendar, then check it against Rahu Kalam for your own city. If the two overlap, skip the overlapping part.
Printed Tamil calendars are calculated for Tamil Nadu. Abroad your sunrise differs, so every slot moves with it. On a 12-hour day starting at 06:00, Monday's Rahu Kalam runs about 07:30 to 09:00; a later winter sunrise in Britain pushes it later.
For everyday tasks such as a phone call or a short errand there is no need to wait. Save Nalla Neram for starts that matter to you.
Good to know
Almanacs differ slightly from one another; if your priest or elders use a particular one, follow it.