FTC carryover is the excess of FTC that you haven't been able to use on your tax returns - usually because the amount that you are able to claim on your return will be restricted to the amount of tax that you would have paid in the US on the same income - as the UK tax rate is higher than the US (on a basic level at least), you'll often have an excess. This excess can be carried over for up to 10 years to use against future foreign income.
Of course, if you are in the UK for future years, you'll probably not be able to use it - as you'll continue to pay more UK tax and therefore continue to gather more FTC carryover. It could come in useful when you're back in the US if you have foreign income going forwards (for example, if the FTC carryover is shown on your 'General Limitation' 1116 form, then you may be able to use some of it to offset your US liability on income relating to any non-US workdays).