In all likelihood, they’re areas near windows and doors, but they might also be next to outlets and light switches on outside walls.
A simple and very inexpensive way to solve the problem is with gaskets made specifically for this purpose. They’re available in most hardware and big box stores, and usually cost less than a dollar each. They’re simple to install: just remove the outlet or light switch cover, select the appropriate gasket, remove the “knockouts” to fit the switch or outlet, press into place, and screw the cover back on over the top. Viola! You’re done, and those few cents will probably save you at least a few dollars over the course of the year.