Code explanation please

the if statements are checking specific characters in B1. if the last char is '.' then the O1 is assigned a value of 1 (no clue why it uses a formula instead of a value) then the dec01 routine is run. if the second last char in B1 is a '.' then O1 equals 2 and dec02 is run and so on
the first If statement is duplicated for some reason and the whole thing looks inefficient to me ;-)

it checks if "." is within the first "x" characters of the right side of "B1". It's done in a queer way though. Then it proceeds to enter some values in "o1"

All those extra Ifs can be removed. Your code can be re-written as -

Sub Calc()

    Sheets("Sheet2").Activate
    ActiveSheet.Unprotect Password:="0000"
    Range("TensionR").Locked = False
    Range("CompressionR").Locked = False
    
    If Left(Right(Range("b1").Value, 1), 1) = "." Then
        Range("o1").Value = "=1"
        Run ("Dec0")
    ElseIf Left(Right(Range("b1").Value, 1), 1) = "." Then
        Range("o1").Value = "=1"
        Run ("Dec0")
    ElseIf Left(Right(Range("b1").Value, 2), 1) = "." Then
        Range("o1").Value = "=2"
        Run ("Dec1")
    ElseIf Left(Right(Range("b1").Value, 3), 1) = "." Then
        Range("o1").Value = "=3"
        Run ("Dec2")
    ElseIf Left(Right(Range("b1").Value, 4), 1) = "." Then
        Range("o1") = 4
        Run ("Dec3")
    ElseIf Left(Right(Range("b1").Value, 5), 1) = "." Then
        Range("o1").Value = "=5"
        Run ("Dec4")
    Else: Range("o1").Value = "=0"
    
    End If
    
    ActiveSheet.Protect Password:="0000"
    
End Sub

I think you should use the function:
INSTR(1, Range("b1"), ".")

in your else part you assign 0 to o1 but then don't do anything else

it would be simpler to use

    Dim lDec As Long
    lDec = InStr(Range("B1").Value, ".")
    If lDec = 0 Then
        Range("O1").Value = 0
    Else
        Range("O1").Value = Len(Mid(Range("B1").Value, lDec + 1))
    End If

to put the number of decimal places in O1. I'm not sure why you need separate routines based on the different number of dp.

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