somebody told me it was a bottom value , but I still don't get the whole picture.
Note: Since there is more than one use case of background in geostatistics, I will pick only one. If this is not the right background you are looking for, please edit and clarify your question.
Answer: A data point can be given by an absolut value
A or a background value
B plus a deviation
B + D = A.
Example: If you are looking at a map showing the mean temperature of year
X, you can look at the absolute field
A or on its deviation
D from the climatological year mean temperature of 1961-1990
B. Sometimes the latter tells you more, depending on what you are interested in. Additionally, analyses and statistical methods are often applied to the deviation field instead of the absolute field to identify patterns.