IP - This stands for ingress protection and is a rating system that's set up by the International Electrotechnical Commission (IEC). So you'll usually see this as IP67, IP68 or something similar.
It's present to determine two things. The first digit represents its ability to ingress (fend off) solid objects like dust. The second digit relates to its ability to ingress liquid. The higher number, the better the protection your wearable should offer against the kind of things that could really do serious damage.
You can check out a chart breaking down the numbers and what the mean here.
ATM - This stands for atmospheres and relates to the measurement used in pressure tests. Those atmosphere measurements are translated into water depth to indicate what a device can withstand. So if you have a smartwatch that promises 5ATM waterproofing, it is able to withstand pressure equivalent of 50 metres depth. Pebble's smartwatches are waterproof (water resistant according to the website) up to 5ATM, for example.
Water pressure, of course, can vary, particularly when swimming, so while you might see a wearable with an ATM rating, it might not actually be safe for immersion. Devices with a 1ATM or 3ATM rating are not considered swim friendly. It's always to double check on the company's website because these ratings can vary.