Basically, if you weren't able to use your capital loss on last year's tax return then you can use it this year. For example, if your taxable income was zero last year (income minus deductions and exemptions) then you didn't get any benefit from the capital loss so the capital loss carries over to this year's return.
Another example would be if your capital loss was more than $3,000 and you didn't have any capital gains to offset against the capital losses. You can only deduct a maximum of $3,000 of capital losses on your Form 1040 each year. Any capital losses more than that carry forward each year until they are all used up.