It's always best to call your doctor about spotting and let them decide how to proceed, especially if there is cramping accompanying it. However the general word I've gotten from my doctor is that brown blood is old blood, and indicates healing, for example from implantation, as Taramum suggested, or from cervical irritation, etc.
I started spotting brown the day my period was due and I haven't had a day without brown spotting, up to and including today. I told my OB the first time it happened, and have gotten extra monitoring and ultrasounds because of it. I have also had a full-on gushing bleed with red blood (9w4d), and baby is still okay. A lot of times, the spotting has no explanation, and it causes no harm, but it should always on the doctor's radar so they can monitor things. They want the chance to advise on lifestyle changes that may help--e.g pelvic rest or lifting restrictions.