what makes the brake light in the dashboard come on in a 99 ford tarus?

[Stéphanie]

I bought a 99 ford tarus a couple of weeks ago and the abs light was on in the dashboard which I was told meant that the abs doesnt work which I was ok with but then last week my red brake light come on and i lost most of my brake pedal. I got the pads changed in the front and the light went off then today I was driving and red brake line came on. I checked my brake fluid and it was a little low so I filled it up but the light is still on. I need some ideas what to check now