1) I want to use SST product but I need to know the limits of its reliability. For example if I have a value of 26 degrees of celcious it means tha the real value is between 25-27 degrees (+-1)????

2) the satellite passes the same time always???

thank you very much for your time
stella

Stella,

If memory serves me (but I wouldn't trust it), MODIS SST accuracy is on the order of +/- 0.25 degrees

The satellite overpass time for Aqua and Terra will vary from day to day, with a 16 day repeat cycle. 
The orbit for the Aqua and Terra platforms are actively maintained to ensure this repeat cycle.
Attached is an example overflight prediction for MODIS-Aqua illustrating this.  Note the time for
Feb 1, 2011 and Feb 17, 2011 (16 days apart) are within 1 minute of each other and the view geometry
is nearly identical as well.

Regards,
Sean