Winston and Julia are living in an environment that forbids them to have a relationship.  They must sneak around to meet and when they do meet they only have a limited amount of time.  When they run into each other they can speak only briefly so they don't arouse suspicion.  Winston describes how Julia can pick up a conversation from the very point that they have left it when previously parting.  She describes this ability as "talking in installments" because they never have a chance to complete an entire conversation.  They speak briefly and then the next time they meet they add another "instalment" to the previous conversation. As we read on page 107,

"...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."