This is for many very common knowledge. However I saw this question recently and thought I would put a post out there with an answer. The question was on how to hold a value when dragging a column or row. Sometimes you have a fixed value in your spreadsheet which you wish to use for every row, but not hardcode it.
Below I have made a small example of a spreadsheet where this has gone wrong. Not all the rows are subtracted by a hundred, only the first one:
I wish to use the value for S1 for every row. Subtracting it from the T in the U column. However it is only applied to the first one when dragging the U1 cell down. The trick to keep using the field S1 is to use the $ sign.
As it can be seen above - using a $ in a formula will hold that value. That is it!
Let me know in the comments, if this helped you :)