In one line
An index is a weighted average of its members' prices, a future on it is a contract to settle that average on a date, and the gap between the two is the basis.
How it works
NIFTY is fifty companies weighted by free-float market value: the number moves because its members do, weighted by size, and nobody trades the index itself. A future on it settles against the index at expiry, and until then it trades a little away from the index — the basis — which reflects the cost of carrying a position to that date. Basis is ordinary, not a signal, and it narrows towards expiry by construction.
The basis is why a comparison between a futures price and an index level has to be done as a CHANGE rather than a difference. GIFT Nifty trades in Gujarat's international centre while India's cash market is shut, so it is the one live number available before the bell. What travels overnight is GIFT's own move against its own previous close — subtracting one price from the other would print the basis as a gap on a morning nothing happened.
Contracts also ROLL. On the session after an expiry the front contract becomes the next month, at a different price for reasons that have nothing to do with the day, so comparing today's front-month close with yesterday's without checking that it is the same contract would show a move nobody made. The desk looks up the previous close for the SAME expiry and prints nothing where there is not one.
What it is not
The basis says nothing about which way the index goes next, and GIFT's overnight move is not the opening level. This desk measured that second one across 782 sessions: applying GIFT's whole move to the previous close missed the actual open by a median of 93 points, where simply assuming a flat open missed by 54. So the desk publishes no indicated level at all — only the direction record, which does carry, and the band of what has actually happened after moves of this size.