HRT is short for hormone replacement therapy. Occasionally to differentiate it from other forms of hormone therapiy it is referred to as cross-sex hormone replacement therapy, but if you're talking to trans folks HRT.

Androgens cause male-associated traits to happen, like tons of muscles and hair on your face and androgenic alopecia (hair loss), estrogens cause things to happen you'd associate with women, like breast development and soft skin.

In short, taking opposite sex hormones (usually combined with drugs that fight the hormones your body creates) is sort of like starting the other puberty, which over time make you look less like your birth-assigned gender and more like the one commonly associated with the suite of hormones you're talking.

MTF folks on HRT generally take anti-androgens and drugs like estradiol in order to stop the effects of testosterone and stuff and make their bodies more consistent with their identities. It can't undo bone growth, it won't generally make things like facial and body hair go away (hair removal is expensive and takes a lof of time), but the effects can be pretty dramatic and satisfaction is genrally pretty high.

There's actually quite a lot of material you're likely to be reading in short order if you're trans and looking at HRT. Take your time. Breathe. You probably have a lot of misconceptions about what trans even means. Not everyone is interested in doing very expensive, risky surgeries on their tackle. Some people are. For a lot of folks it's a medical necessity. Trans experiences are really broad and the only thing you need to worry about is what you feel is right for you and making sure you have the tools to navigate some very heavy stuff at your own pace.