The Total column shows the total number of people in that county or town with this surname. For example, there were 68 people called MACISAAC in Inverness-shire at the time of the 1881 census.
The Frequency column shows the percentage of people in this county or town with this surname. For example, a frequency of 0.0779 in Inverness-shire means that 0.0779% of the people in Inverness-shire on census day were called MACISAAC.
The Index column shows the relative probability of finding someone called MACISAAC in this county or town, compared with the probability of finding them anywhere in Britain as a whole. An index of 1 means that if you pick someone at random from this county or town, you have exactly the same probability of picking someone called MACISAAC as if you picked at random from the whole of the UK. Where the index is higher than 1, then you are more likely to find someone called MACISAAC here than if you picked from the UK as a whole, and where it's lower then you are less likely. The actual figure shows the level of probability - for example, a figure of 2 would indicate that you are twice as likely to find someone called MACISAAC here than in the UK as a whole, and 10 would make it ten times as likely. The value of 288.26 in Inverness-shire means that you are 288.26 times as likely to find someone with the surname of MACISAAC in Inverness-shire than you would be in the whole of the UK.