The important phrase here is “settled upon her”. The appropriate sense is:
settle, v. V.30.b. To secure (property, succession) to, on or upon (a person) by means of a deed of settlement.
Oxford English Dictionary.
The effect of a deed of settlement is that the beneficial owner of the property (here, Lydia Wickham) is not allowed to sell it or dispose of it in her will. She will receive the income from the property during her life, and the property will pass to her children upon her death. The total property thus settled will amount to £2,000. If the assets are able to realise five per cent, then her annual income will be £100: enough to live on, if the Wickhams are very frugal, or to supplement Mr. Wickham’s army salary. (A farm labourer of the period might earn about £20 a year.)
The reason that the property was ‘settled’ on Lydia in this fashion, rather than simply given to her, was to prevent Mr. Wickham from squandering it. Prior to the Married Women’s Property Act (1870) a woman in England lost control over her property when she married. Her personal property became her husband’s, and she could not rent or sell her land without her husband’s permission. Mr. Darcy knows Mr. Wickham’s character too well to trust him to look after Lydia’s property without tying it up in law.
The £2,000 of property settled on Lydia is worked out as follows. “Another thousand in addition” comes from Mr. Darcy, and the first £1,000 are “her own”. But what “her own” actually means is her prospective share of Mrs. Bennet’s marriage portion. Mr. Gardiner writes to Mr. Bennet as follows (chapter 49):
All that is required of you is, to assure to your daughter, by settlement, her equal share of the five thousand pounds secured among your children after the decease of yourself and my sister; and, moreover, to enter into an engagement of allowing her, during your life, one hundred pounds per annum.
The narrator confirms this:
Five thousand pounds was settled by marriage articles on Mrs. Bennet and the children.
This property is also tied up, by a marriage settlement, so that Mr. Bennet cannot sell it to realise the capital. Instead, he must pay Lydia her share of the proceeds during his lifetime, and arrange in his will to settle Lydia’s share of the property on her upon his (and his wife’s) death. The sum of £100 per year seems rather more than the realistic returns on £1,000 of property, so presumably it includes an element of personal allowance.