What is it mean when your fish is laying on its side?

Probably because it is dead or dying. All fish including Bettas die when they are not looked after properly. The basic rules for keeping any kind of fish successfully are :- 1 inch of fish needs at least 1 gallon of water. :- Every tank needs a permanently running cycled filter. :- Every tank needs at least 50% of its water changed every week. Bettas are a 3 inch fish so will need at least a 4 gallon tank. They are tropical so they will also need a heater/thermostat set at 78F and a thermometer so that you can check the temperature daily. They also need to be fed morning and evening as much Betta food as they can eat in 15 minutes. No food should ever be left to rot in the tank because it will poison the water. If you follow the above guidelines your fish should survive OK. If you fail in any of the requirements above I can guarantee that your fish will be constantly getting sick and will live a shortened lifespan..