Sex is how you are born. Your chromosomes and physical appearance without any transition therapies. There are three recognized sexes, Male Female and Intersex.
Gender is how your brain is wired, usually this matches up with your sex on the spectrum but in some cases it doesn’t. Brains scans have found that transgender individuals brain’s are wired like their gender identity, and not their sex.
If your gender and sex match up you are cis, and if they don’t then you are trans.