Never underestimate how wet the inside of your pack can get. For example: It rains during the night, and then next day, you are packing a wet tent. Unless you have a waterproof compression sack, things may get wet inside your pack.
There are some different ways to keep things dry.
Ziplock bags can be used to keep smaller items dry. They are most often used for phones, cameras and cash. As ziplock bags are clear you can often still use your phone whilst they are kept in the bag, useful if it is raining. Make sure there are no holes in the bag and they are fully sealed.
A rain gives your pack some water resistance- enough to keep your kit dry should it rain. Larger rucksacks will come with a rain cover attached but are easy to obtain if not. They are generally elasticated around the edge to ensure that your bag is kept completely covered.
Ultimately, few backpacks are 100% waterproof, but if you’re looking for a bag that will keep your kit dry even when fully submerged under water, it’s worth considering these features:
Single compartment packs
Choosing a backpack with a single compartment means there are less zips where water can penetrate (as most rucksacks do not have waterproof zips.)
Roll top opening
This helps to keep the water out in a similar design to a dry bag. It provides an airtight seal to keep your gear dry. Fully waterproof rucksacks will have a roll top in place of a zip.