The past is a time period that has already happened. Subjectively, this is all events which occurred before one's present time.
In 2368, faced with the dilemma of the planet Penthara IV slowly turning to ice which would result in thousands, maybe tens of thousands of deaths, in a matter of weeks, Captain Jean-Luc Picard sought the help of time traveler Berlinghoff Rasmussen, who was at the time believed to be from the 26th century. Rasmussen was unwilling to influence Picard as it could change everything in that sector, in that quadrant of the galaxy, causing Rasmussen's history to unfold in a way other than it already had. Picard stated that perhaps he didn't care about Rasmussen's past, because Rasmussen's past was Picard's future and hadn't been written yet. (TNG: "A Matter Of Time")