Rashi is silent on the meaning of the word, and explains Rachel's motive simply as "she wanted to distance her father from idolatry." The Rach explains "For he would say, 'a god who can be stolen is nothing.' "
Rasag simply glosses the word: "hateraphim: pesalim."
With Rashbam we start to get a refinement in definition. He explains that teraphim is a more specific term than pesalim, and refers to idols used for divination. The word also appears in Hosea 3:4 in conjunction with the term ephod, and in Zecharya 10:2 in a similar sense. Lavan, he explains, would regularly consult them.
Chizkuni goes into Rachel's motive: "And Rachel stole the teraphim --- so that they would not disclose the trail (lit: scent) of Jacob." Radak says much the same thing: "The teraphim were made to tell the fate of man, to receive the power of the heavens, and Rachel stole them so that they would not reveal to her father which road they had taken." If the teraphim were not just ordinary idols but were considered oracular, and if Rachel still believed in them, then this would seem a reasonable precaution. (I don't want her to have believed in their power, but I'm projecting my own beliefs onto her and I shouldn't do that.)
Ra'ava has a more detailed explanation: "There are those who say that the teraphim were instruments of copper, constructed [by those] with an understanding of the division of hours. And others say that through the power of the wisdom of the stars and their position at such-and-such an hour it had the knowledge to tell one's fate." In other words, some sort of astrolabe used for astrological divination. This would make it not an idol in the literal sense, although still certainly used for idolatrous purpose. He brings down various additional prooftexts and examples, which are worth reading, but the hour is late and I'm not up to translating them all.
I find the idea that the teraphim were astrolabes used for astrological divination compelling, but I'm still troubled by the idea that Rachel expected her father to be successful in using them against Jacob. Here's another possible interpretation: Rachel knew that her father would consult the teraphim before beginning his pursuit of Jacob. And even though she knew them to be false oracles of false gods, she also knew that when her father couldn't find them, he'd take the time to look for them and, failing that, he'd go to his neighbor to borrow the neighbor's teraphim. By this reading, Rachel's motive was quite simply tactical: she knew her enemy and was playing for time.