This is the percentage of the property value you're loaned as a mortgage – in other words, the proportion you're borrowing.
To calculate this, simply subtract your deposit/equity as a percentage of the property value from 100%. So if you've a £60,000 deposit on a £300,000 home, that's a 20% deposit. This means you owe 80% – so the LTV is 80%.
Read more on how to calculate your LTV.