Milk replacer is a powder that contains the protein, fat and vitamins that lambs would normally get from their mothers milk. The powder is mixed with warm water and put into a bottle with a nipple that the lambs suck to drink. When they are young they get a bottle often as they have small stomachs that fill up fast and they get hungry quickly.
It always pays to be prepared, ideally during lambing you won't need to bottle feed any lambs but just incase it's good to have a bag. We also milk ewes that produce a lot of milk or lose their lambs and freeze so it can be warmed up and fed later. Lambs need to be bottle fed if their mom can't produce enough milk on their own, dies or doesn't accept the lamb as her own. We find that our bottle lambs tend to not grow as well as other lambs and so we try to start them on creep feed early. Last year we had 4 bottle babies (a ewe with triplets got mastitis and one ewe didn't accept one of her triplets), hopefully this year we will have good luck and not have any bottle lambs.