Julia, in any case, seldom had an evening completely free. She spent an astonishing amount of time in attending lectures and demonstrations, distributing literature for the junior Anti-Sex League, preparing banners for Hate Week, making collections for the savings campaign, and such-like activities. It paid, she said, it was camouflage. If you kept the small rules, you could break the big ones. She even induced Winston to mortgage yet another of his evenings by enrolling himself for the part-time munition work which was done voluntarily by zealous Party members.
The whole quotation is "If you kept the small rules, you could break the big ones." The context of the quotation describes Julia's false activities within the Big Brother party. Julia has no commitment to the Party but does have a commitment to her freedom to live as she chooses within a restrictive environment that regulates virtually every step she takes and virtually every word she speaks. A description of this hyper-regulation precedes the the quotation you ask about. The narrator describes how Julia escapes the hyper-regulation--and teaches Winston to do so too--through clandestinely stolen seconds filled with such tricks as "talking by instalments":
... they could meet only in the streets, in a different place every evening and never for more than half an hour at a time. In the street it was usually possible to talk, after a fashion. As they drifted down the crowded pavements, not quite abreast and never looking at one another, they carried on a curious, intermittent conversation which flicked on and off like the beams of a lighthouse, suddenly nipped into silence by the approach of a Party uniform or the proximity of a telescreen, then taken up again minutes later in the middle of a sentence, then abruptly cut short as they parted at the agreed spot, then continued almost without introduction on the following day. Julia appeared to be quite used to this kind of conversation, which she called 'talking by instalments'. She was also surprisingly adept at speaking without moving her lips ...
Julia's philosophy of "[keeping] the small rules, [so you can] break the big ones" succinctly expresses a subversive lifestyle that gives the appearance of being zealously dedicated to political oppression by doing unimportant "small" things, like attending lectures, making banners, going to demonstrations, while really living in a way that violates the most important ideas imposed by the oppressors, like having sexual encounters secreted away on a hikers' route and conversational rendezvous in the midst of crowds. In other words, activities in accord with "small rules" provides, as she said, "camouflage" for a life that opposes the "big" rules in every regard: "It paid [money], she said, it was camouflage. If you kept the small rules, you could break the big ones."
She would be hanging about among the stalls, pretending to be in search of shoelaces or sewing-thread. If she judged that the coast was clear she would blow her nose when he approached; otherwise he was to walk past her without recognition. But with luck, in the middle of the crowd, it would be safe to talk for a quarter of an hour and arrange another meeting. ... She flung herself into his arms, kissed him almost violently, and a moment later pushed her way through the saplings and disappeared into the wood with very little noise.
She and Winston eventually pay a big personal price for breaking the "small rules" once their violation of the "big ones" is discovered and their "camouflage" is seen through, but, for a time, it provides them with a sense of freedom, independence and human autonomy, no matter how fleeting, insignificant, and hard to capture that sense is.
In Chapter III of Part II of 1984, the narrator says, "If you kept the small rules, you could break the big ones." This is referring to Julia's plan to keep Winston involved in Big Brother activities while continuing their illegal affair. This is one of many times when the philosophical differences between Julia and the protagonist Winston can be seen. This quote highlights Julia's ability to undermine the Party from within. She shows no desire to overthrow Big Brother as Winston does. She is satisfied to function within Big Brother's regime, to superficially do as the Party asks while rebelling in her own ways. She is rather apathetic towards Winston's dream of ridding the world of Big Brother's powers and being completely free.